The reactions between pyridoxal phosphate (PLP) and ethylamine (Et) and ethylenediamine (Etd) were studied in buffered aqueous solution. The formation of Schiff bases by reversible reaction was observed in each case according to the electrochemical results. The presence of a cyclic geminaldiamine and/or enol tautomer of the Schiff base was detected in the reaction of PLP with Etd by comparing polarographic and voltammetric results of both reactions. The increase in stability of the Schiff base as diprotonated species, as well as the difference in which nitrogen on the Schiff base undergoes a fast protonation are associated with the presence and reduction of the enol tautomer. This implies a deviation from normal behaviour of the PLP Schiff base in aqueous solution. The results suggest that a very fast conversion of geminaldiamine to Schiff base takes place at the electrode with monoprotonated and unprotonated species. The presence of a second amino group in the ethylenediamine moiety of the Schiff base PLP-Etd essentially does not modify the ECE-Disp2 mechanism in strong basic media.