The reactions of 5-benzylidene-3-phenylrhodanine (2; rhodanine = 2-thioxo-1,3-thiazolidin-4-one) with diazomethane (7a) and phenyldiazomethane (7b) occurred chemoselectively at the exocyclic C=C bond to give the spirocyclopropane derivatives 9 and, in the case of 7a, also the C-methylated products 8 (Scheme 1). In contrast, diphenyldiazomethane (7c) reacted exclusively with the C=S group leading to the 2-(diphenylmethylidene)-1,3-thiazolidine 11 via [2+3] cycloaddition and a two-fold extrusion reaction. Treatment of 8 or 9b with an excess of 7a in refluxing CH2Cl2 and in THF at room temperature in the presence of [Rh2(OAc)4], respectively, led to the 1,3-thiazolidine-2,4-diones 15 and 20, respectively, i.e., the products of the hydrolysis of the intermediate thiocarbonyl ylide. On the other hand, the reactions with 7b and 7c in boiling toluene yielded the corresponding 2-methylidene derivatives 16, 21a, and 21b. Finally, the reaction of 11 with 7a occurred exclusively at the electron-poor C=C bond, which is conjugated with the C=O group. In addition to the spirocyclopropane 23, the C-methylated 22 was formed as a minor product. The structures of the products (Z)-8, 9a, 9b, 11, and 23 were established by X-ray crystallography.