Stability and electrocatalytic properties for the hydrogen evolution reaction (h.e.r.) were investigated on pure and polypyrrole (PPy)-coated GdNi4Al intermetallic alloy in 1 M NaOH electrolyte solution. In both cases, a surface film played an essential role in controlling this behavior. GdNi4Al was produced by arc melting of its pure components. Electronically conducting PPy film of 0.86 gm thickness was electrosynthesized on the alloy surface. A segregation phenomenon driven by preferential Gd oxidation, because of high reactivity of rare earths in contact with aqueous media, resulted in enrichment of the surface film with metallic Ni. The intermetallic's high catalytic activity for h.e.r. was ascribed to formation of a continuous, electronically conducting net-like structure of nickel nanoparticles in the matrix of a barrier Gd2O3 surface film, facilitated by the formation of interfacial Raney-type structure by leaching of Al in a basic solution, pH 14. In the narrow overpotential region, the PPy coating on the surface of the GdNi4Al electrode prevented the formation and growth of surface oxides and the loss of metallic conductivity, which, as a consequence, resulted in a smaller Tafel slope (Deltab(c) = -74 mV) and greater catalytic activity for h.e.r. At overpotentials higher than -0.12 V, the PPy layer did not protect the surface of the GdNi4Al electrode from mechanical decomposition, due to extensive hydrogen penetration in the bulk of the alloy. Structural characteristics of surface films formed on the GdNi4Al alloy as well as at the solid-liquid interface were investigated in situ using structural sensitive values of capacitance and resistance obtained by the complex nonlinear least-square (CNLS) method used in the fitting procedure of impedance spectra recorded in the frequency range 100 kHz-10 mHz. (C) 2003 Elsevier Ltd. All rights reserved.