The title compound, C(10)H(15)N(2)O(4) (+)·Cl(-), was obtained as an unexpected by-product during the synthesis of 1-[2-(substituted ar-yl)]-3-methyl-pyrazol-5-ones. The hydrazide group is essentially planar, with an r.s.m. deviation of 0.020 (2) Å, and is oriented at a dihedral angle of 30.52 (3)° with respect to the benzene ring. In the crystal structure, the cations and anions are linked through N-H⋯O and N-H⋯Cl hydrogen bonds, forming a mol-ecular tape running along the b axis.