1,3-Dipolar cycloadditions of N-methyl-C,C-diphenylnitrone (15) and N-methyl-C-phenylnitrone (16) with aliphatic thioketones are equilibrium reactions. The 1,4,2-oxathiazolidines were characterized and their dissociation constants measured by H-1 NMR analysis and by visible spectrophotometry. K-diss Of the adduct 28 from 16 and 2,2,6,6-tetramethylcyclohexanethione (27) was determined from 20-76 degrees C, revealing Delta H-add = -10.8 kcal mol(-1) and Delta S-add = -28 eu. The inertness of diaryl thioketones vs nitrones has thermodynamic reasons. According to rate measurements with 16, the activity of the highly hindered thione 27 exceeds 5-fold that of dimethyl acetylenedicarboxylate (DMAD), the top dipolarophile with a CC multiple bond; the cycloaddition to adamantanethione-despite adverse steric effects-is 1500 times faster than that to DMAD. Rate constants for the cycloaddition of 16 to 2,2,4,4-tetramethyl-3-thioxocyclobutanone (10) were measured in 12 solvents. The small and slightly inverse relation to solvent polarity rules out a zwitterionic intermediate but is consistent with a concerted pathway.