The (silylethynyl)- and the (tolylethinyl)-hydrazines 1a and 1b react with aryl isocyanates 2a-e and arylsulfonyl isocyanates 2f,g to form the corresponding substituted 3-arylimino-(or arylsulfonylimino-) 1,1,2-trimethyl-5-oxopyrazolidinium-4-ides 3a-h. The silyl group in the 4-position can be replaced by a hydrogen atom (formation of 6a-e). Heating of 6d in toluene induced the unequivocal transformation of the betaine to an isomer, the spectrocopic data of which are in concordance with those of the 4-quinolone derivative 7. Aroyl isocyanates 8 react in a [4+2]cycloaddition with la to furnish the 1,3-oxazin-4-ones 9a and b. The stannylated ynehydrazine 1c adds the cumulenic system of an aryl isocyanate (2d) across the C_sp-Sn single bond, and trimethylstannyl .N-(p-chlorophenyl)-3-(trimethylhydrazino)propiolimidate 10 is isolated. With two molecules of 2d the hydantoin derivative 11 is formed.