The protonation site of some alkyl-(MeNHNH2 and Me2NNH2) and aryl-(ArNHNH2, Ar = Ph, 4-OMeC6H4, 4-NO2C6H4) hydrazines has been investigated in water, methanol, and dimethyl sulfoxide as solvents by measuring the change in the relaxation rate of N-14, N-15, and C-13 between the neutral and protonated forms. The relative stability of the two protonated forms was also investigated theoretically by means of semiempirical and ab initio calculations. The preferred protonation site of 1,1-dimethylhydrazine is the alkylated nitrogen, whereas methylhydrazine may also undergo protonation at the unsubstituted nitrogen. Phenylhydrazine and 4-nitrophenylhydrazine are protonated at the primary amine nitrogen, whereas 4-methoxyphenylhydrazine undergoes protonation at both nitrogens.