The electrochemical deposition of Ni and Co on GaAs and GaP at pH 5 was investigated by means of cyclic voltammetry and rotating disk voltammetry. The current-potential behavior of the two semiconductors in dilute metal ion solutions appeared to he different: at n-GaP, a stepwise reduction of the metal ions was observed whereas at n-GaAs this was not the case This observation can be explained on the basis of two alternative reaction mechanisms for the reduction of the monovalent intermediate. Under well-chosen conditions of deposition potential and metal ion concentration, it appeared to he possible to form adherent n-GaAs/Co, n-GaAs/Ni, and n-GaP/Co Schottky contacts with good rectifying properties. The barrier height Phi(B) of the n-GaAs/Ni contacts appeared to increase as a function of time, leading to values of Phi(B) which were dependent on the deposition potential V-D. At n-GaAs/Co and n-GaP/Co junctions, this aging effect was absent, leading to values of the barrier height which were independent of V-D. It was further observed that the occurrence of side reactions during metal deposition may exert a great influence upon the properties of the metal/semiconductor junctions formed.