The solvent extraction of uranium(VI) and thorium(IV) from sodium nitrate solutions (0.20-6.00 M) by a series of dialkyl sulphoxides with different structures was studied. For sulphoxides with n-alkyl groups (R2SO, where R = n-hexyl, n-octyl and n-decyl) using 0.20 M solutions in xylene, the extractions of both uranium and thorium are relatively high, and the values of the separation factor beta(Th)(U) are correspondingly low (approximate to 20). Replacement of an n-hexyl group by a cyclohexyl group has little effect on metal extraction, whilst the introduction of a second cyclohexyl group causes a slight decrease in extraction. Similarly, there is little variation in the extraction of uranium and thorium through the series of asymmetrical compounds RR’SO, where R=n-octyl and R’=cyclopentyl, cyclohexyl or cyclooctyl. When two aromatic (phenyl) rings are introduced into the sulphoxide, however, the extraction of both metals falls to zero. For the series of isomeric compounds R2SO with C-8 alkyl groups, the separation factors increase in the order : R = n-octyl, 2-ethylhexyl, 2-octyl, 3-octyl, which is also the order of increasing steric bulk of the alkyl group. For these compounds, slope analysis studies are consistent with the formulation of the extracted metal complexes as UO2(NO3)(2)(R2SO)(2) and Th(NO3)(4)(R2SO)(3).