C 1s, N 1s, O 1s, Fe 2p and Mo 3d X-ray photoelectron core-level spectra of newly synthesized iron and molybdenum naphthalocyanines have been recorded following exposure to air for different times. The observed modifications of the C 1s and N 1s lines provide information on the electronic structure perturbations related to dioxygen fixation, and the intensity and shape of the O 1s, Fe 2p and Mo 3d peaks provide information on the rates of oxidation, the coordination number of the central metal ion and the kinetics of oxygen penetration into the bulk. From these X.p.s. results, new methods are suggested for the synthesis of compounds of electrochemical interest.