Zinc phosphide Zn3P2 has been intensively investigated as one of the most promising materials for applications in low-cost solar energy converters and broad- range photodetectors. The Schottky barriers formed by Al on Zn3P2 p-type crystals have been studied. Substantial differences were observed depending on whether the semiconductor surface was chemically etched, mechanically polished or/and heat treated at 523 K. Measurements of current-voltage characteristics and photovoltaic spectrum of Al-Zn3P2 contacts at room temperature have been used for determination of some electrical parameters of junctions as well as optical transitions and hole concentration in the semiconductor. The value of barrier height, (Phi) B, changed from 0.76 to 0.78 eV, the junction depth--from about 3 to 79 micrometers , and the contact resistance--from 5.9 X 103 to 858 X 103 (Omega) . The hole concentration of examined polycrystalline samples was equal to 2 X 1013 divided by 5 X 1015 cm. The condition of semiconductor surface seems to have an essential influence on obtained electrical parameters of Al-Zn3P2 junctions and their spectral characteristics. To understand the problems connected with metal-Zn3P2 interface layer formation, the free enthalpy, (Delta) GR, was calculated. The analysis of the results, obtained especially for the Mg and Al, have indicated that Al is also a good candidate for formation of rectifying contacts to Zn3P2.