Self-assembled monomolecular films of 1,8,15,22-tetraaminophthalocyanatocobalt(II) (4 alpha-Co(II)TAPc) and 2,9,16,23-tetraaminophthalocyanatocobalt(II) (4 beta-Co(II)TAPc) on Au surfaces were prepared by spontaneous adsorption from solution. These films were characterized by cyclic voltammetry and Raman spectroscopy. Both the surface coverage (Gamma) and intensity of the in-plane stretching bands obtained from Raman studies vary for these monomolecular films, indicating different orientations adopted by them on Au surfaces. The 4 alpha-CO(II)TAPc-modified electrode exhibits an E-1/2 of 0.35 V, while the 4 beta-Co(II)TAPc-modified electrode exhibits an E-1/2 of 0.19 V, corresponding to the Co-II/Co-III redox couple in 0.1 M H2SO4. The Gamma estimated from the charge associated with the oxidation of Co(II) gives (2.62 +/- 0.10) x 10(-11) mol cm(-2) for 4 alpha-Co(II)TAPc and (3.43 +/- 0.14) x 10(-10) mol cm(-2) for 4 beta-Co(II)TAPc. In Raman spectral studies, the intensity ratio between in-plane phthalocyanine (Pc) stretching and the Au-N stretching was found to be 6.6 for 4 beta-Co(II)TAPc, while it was 1.6 for 4 alpha-Co(II)TAPc. The obtained lower Gamma and intensity ratio values suggest that 4 alpha-Co(II)TAPc adopts nearly a parallel orientation on the Au surface, while the higher Gamma and intensity ratio values suggest that 4 beta-Co(II)TAPc adopts a perpendicular orientation. The electrochemical reduction of dioxygen was carried out using these differently oriented Pc's in phosphate buffer solution (pH 7.2). Both the Pc's catalyze the reduction of dioxygen; however, the 4 alpha-Co(II)TAPC-modified electrode greatly reduces the dioxygen reduction overpotential compared to 4 beta-Co(II)TAPc-modified and bare Au electrodes.