Dialkyl (1-cyano-2,2-bismethylsulfinylvinyl)phosphonates, (R1O)2P(O)C(CN):C(SMe)2 (R1 = Et, CHMe2) react with hydrazines, R2NHNH2 (e.g., R2 = H, Ph) to yield pyrazoles I. The compn. of one reaction product, i.e., I (r1 = Et, r2 = CHMe2) was examd. by x-ray crystal structure anal. N-unsubstituted pyrazoles I (R1 = Et, CHMe2, R2 = H) treated with 1,3-diketones, R3C(O)CHR4C(O)R3 (e.g., R3 = Me, R4 = H, Me) give pyrazolo[1,5-a]pyrimidines II.