Platinum overlayers on InP(110) and GaP(110) have been studied by photoelectron spectroscopy and measurement of current-voltage (I-V) and capacitance-voltage (C-V) characteristics. The reaction processes at these interfaces were monitored in parallel with the Fermi level movement in the gap up to the formation of the interfacial potential barrier, φb. Synchrotron radiation photoemission studies reveal the reaction between Pt and phosphorus, and also the alloying of indium and gallium with the Pt layer. For both surfaces, the band bending measurement is affected by a surface photovoltage caused by the photoemission light source. I-V (and C-V) measurements were carried out in situ, at different substrate temperatures in order to evaluate the bulk barrier height, and to obtain an experimental value for the Richardson constant. The values of φb(0.84 V for p-GaP(110) and 0.53 V for n-InP(110)) are in agreement with the values deduced from the photoemission measurements. These results are compared with data for other metals and are discussed in the context of current theories of barrier formation.