| MolarMass = 264.04037&nbsp;g/mol<ref name=crc1/>

| Appearance = white solid<ref name=crc1/>

| Density = 10.000&nbsp;g/cm³<ref name=crc1>Haynes, p. 4.95</ref>

| Solubility = insoluble<ref name=crc1/>

| SolubleOther = insoluble in [[alkali]]<br>slightly soluble in [[acid]]<ref name=crc1/>

| MeltingPtC = 33903350

| RefractIndex = 2.200 (thorianite)<ref>Haynes, p. 4.144</ref>

| MagSus = &minus;16.0·10⁻⁶&nbsp;cm³/mol<ref>Haynes, p. 4.136</ref>

| LattConst_a = 559.74(6) pm<ref name=Yamashita>{{Cite journal | title = Thermal expansions of NpO₂ and some other actinide dioxides | journal = J. Nucl. Mater. | volume = 245 | issue = 1 | year = 1997 | pages = 72–78 |author1=Yamashita, Toshiyuki |author2=Nitani, Noriko |author3=Tsuji, Toshihide |author4=Inagaki, Hironitsu | doi = 10.1016/S0022-3115(96)00750-7 | postscriptbibcode = 1997JNuM..245...72Y }}</ref>

'''Thorium dioxide''' (ThO₂), also called '''thorium(IV) oxide''', is a crystalline solid, often white or yellow in color. Also known as '''thoria''', it is produced mainly as a by-product of [[lanthanide]] and [[uranium]] production.<ref name=Yamashita/> [[Thorianite]] is the name of the mineralogical form of thorium dioxide. It is moderately rare and crystallizes in an isometric system. The melting point of thorium oxide is 3300&nbsp;°C – the highest of all known oxides. Only a few elements (including [[tungsten]] and [[carbon]]) and a few compounds (including [[tantalum carbide]]) have higher melting points.<ref>{{cite book | last = Emsley | first = John | title = Nature's Building Blocks | edition = Hardcover, First | publisher = [[Oxford University Press]] | year = 2001 | pages = 441 | isbn = 978-0-19-850340-78 }}</ref> All thorium compounds are radioactive because there are no stable [[isotopes of thorium]].

Thorium dioxide is more stable than [[thorium monoxide]] (ThO).<ref>{{cite journal |first1= Heming |last1= He |first2= Jaroslaw |last2= Majewski |first3= David D. |last3= Allred |first4= Peng |last4= Wang |first5= Xiaodong |last5= Wen |first6= Kirk D. |last6= Rector |title= Formation of solid thorium monoxide at near-ambient conditions as observed by neutron reflectometry and interpreted by screened hybrid functional calculations |journal= Journal of Nuclear Materials |volume= 487 |year= 2017 |pages= 288–296 |doi= 10.1016/j.jnucmat.2016.12.046 |bibcode= 2017JNuM..487..288H }}</ref> Only with careful control of reaction conditions can oxidation of thorium metal give the monoxide rather than the dioxide. At extremely high temperatures, the dioxide can convert to the monoxide either by a [[disproportionation reaction]] (equilibrium with liquid thorium metal) above {{convert|1850|K|°C °F}} or by simple dissociation (evolution of oxygen) above {{convert|2500|K|°C °F}}.<ref>{{cite journal |title= The Reaction Occurring on Thoriated Cathodes |first1= Michael |last1= Hoch |first2= Herrick L. |last2= Johnston |journal= J. Am. Chem. Soc. |year= 1954 |volume= 76 |issue= 19 |pages= 4833–4835 |doi= 10.1021/ja01648a018 }}</ref>

Thoria dispersed nickel finds its applications in various high temperature operations like combustion engines because it is a good creep resistant material. It can also be used for hydrogen trapping.<ref>{{cite book | url = https://books.google.com/books?id=iQQcERxsNywC&pg=PA473 | page = 473 | isbn = 978-0-471-43623-2 | title = An Introduction to Materials Engineering. and Science for Chemical and Materials. | author1 = Mitchell, Brian S | year = 2004 }}</ref><ref>{{cite journal | first = Wayne M. | last = Robertson | title = Measurement and evaluation of hydrogen trapping in thoria dispersed nickel | journal = Metallurgical and Materials Transactions A | volume = 10 | issue = 4 | year = 1979 |doi = 10.1007/BF02697077 | pages =489&ndash;501 | postscriptbibcode = 1979MTA....10..489R }}</ref><!-- 10.1007/BF00604042 10.1007/BF00614617 10.1007/BF00551849-->

Thorium dioxide has almost no value as a commercial catalyst, but such applications have been well investigated. It is a catalyst in the [[Ruzicka large ring synthesis]]. Other applications that have been explored include [[Cracking (chemistry)|petroleum cracking]], conversion of [[ammonia]] to [[nitric acid]] and preparation of [[sulfuric acid]].<ref name=Ullmann>Stoll, Wolfgang Stoll(2012) "Thorium and Thorium Compounds" in ''Ullmann's Encyclopedia of Industrial Chemistry 2012''. Wiley-VCH, Weinheim. {{DOI|10.1002/14356007.a27_001}}</ref>

Thorium dioxide was the primary ingredient in [[Thorotrast]], a once-common [[radiocontrast agent]] used for [[cerebral angiography]], however, it causes a rare form of cancer (hepatic [[angiosarcoma]]) many years after administration.<ref>[https://radiopaedia.org/articles/thorotrast Thorotrast]. radiopaedia.org</ref> This use was replaced with injectable [[iodine]] or ingestable [[barium sulfate suspension]] as standard [[X-ray]] contrast agents.