The CS double bond displays an unusually high reactivity in a number of 1,3-dipolar cycloadditions. Ab-initio calculations on different levels are performed for cycloadditions of the parent nitrone to thioformaldehyde and ethylene, respectively. An orientation complex (OC) is found for both cycloadditions. In the case of thioformaldehyde, the nature of the complex is of charge-transfer type as revealed by a perturbational analysis, whereas for ethylene as dipolarophile, the van der Waals character predominates. The transition structures (TS) for cycloaddition resemble closely the OC with a shorter separation and a stronger distortion of the reactants. A negative energy of activation relative to the reactants (e.g., -2.5 kcal mol(-1) (Becke3LYP)) and a small positive barrier (+1.2 kcal mol(-1)) relative to the OC is calculated for thioformaldehyde as dipolarophile; the corresponding values for ethylene are +13.7 and +15.5 kcal mol(-1) respectively. The perturbational analysis shows a strong HOMO(nitrone) - LUMO(thioformaldehyde) interaction as the principal reason for the high thione reactivity. The observed equilibria in the cycloadditions of N-methyl-C,C-diphenylnitrone to 2,2,4,4-tetramethyl-3-thioxocyclobutanon and of N-methyl-C-phenylnitrone to 2,2,6,6-tetramethylcyclohexanethione are found to be well reproduced by PM3 calculations.