A study of the reduction of pyridoxal-5′-phosphate Schiff base with n-hexylamine at a d.m.e. in a wide pH range was carried out. The effects of pH and amine and ethanol concentrations on the d.c. and d.p.p. were studied. The equilibrium formation constant of the Schiff base in aqueous solutions for the entire pH range was calculated. The results found show no direct influence of the ionization equilibria of the Schiff base on the reduction potential of the different species present in the aqueous solution. The effect of ethanol concentration on the formation constant are discussed. Evidence of the influence of protonation equilibria in ethanol-water solutions was obtained by d.p.p.