Abstract The sydnones 1a–c react with the 4, 5-dihydro-v-triazoles 2a–c by long refluxing at 110–140 C affording via non-isolable adducts with elimination of CO 2 and N 2 and rearrangement a mixture of 4, 5-dihydropyrazoles 3a–c, 3-pyrazolecarbaldehyde anils 4a–d, and 3-pyrazolecarbaldehydes 5a–c. Compounds 3 can be deaminated to give 4 which can be hydrolyzed to yield the 3-pyrazolecarbaldehydes 5.–The sydnones 6a, b react similarly ...