Electronic spectrophotometry has been employed to study the mechanism of transimination in bis(N-alkylsalicylaldiminato)nickel(II) complexes (alkyl = Me, Et, Pr(i), Pr(n) or Bu(n)) by ammonium ion in acetonitrile. The reaction is clearly biphasic: fast addition of ammonium and slow elimination of alkylammonium ions. The addition reaction involving the two azomethine bonds of the complex occurs by two consecutive processes and a two-term rate law was found for R = Bu(n). In the successive elimination reaction only one rate constant was observed which depends on the pK(a) and steric strain of the leaving alkylammonium ions.