Department of Chemistry, Inorganic Chemistry Division, University College of Science, Calcutta 700 009 Manuscript received 10 May 1975; accepted 11 June 1976 The polarographic behaviour of Ni(H2O)62+ , NI(NH3)4(H2O)22+, Ni(en)32+, Ni(gly)2, Ni(acac)2, Ni(II)-EDTA complex and Ni(bigH)22+ in 0.1M aqueous KNO3 medium has been studied. Each species has been found to undergo a diffusion controlled two-electron reduction at the DME; the process Is irreversible for all the complexes except Ni(bigH)22+ which undergoes reversible reduc­tion. For the irreversible systems (except Ni(acac)2 where there are other complications) the values of the rate constant, kr, of the electrode process at a reference potential of —1.0 V vs. SCE have been evaluated from polarographic characteristics following known methods. From the kr values of each system at four different temperatures in the range of 25° to 40° the corresponding activation energy, Ea, has also been evaluated. Results indicate that for complexes of charge type 2+ the ease of reduction increases in the sequence of decreasing Dq value of the complex, while for complexes of the same Dq value the ease of reduction follows the sequence cationic > non­ionic > anionic. A plausible mechanism of electron transfer has been discussed.