Abstract The polarographic reduction of 4-arylhydrazone-N′-benzysulphonyl-3-methyl-2-pyrazoline-5-ones gave two, two-electron transfer, well defined, well separated, diffusion controlled, irreversible waves in B.R. buffers of pH range 7.0–11.0. In acidic medium the E 1/2 values of the two waves are so close that only one 4-electron transfer, well defined, diffusion controlled, irreversible waves is obtained. The reduction in these compounds takes place at the-NH-N=C-bond. The effect of substituents and its correlation with the Hammett substituent constant (σ) have also been studied.