The effect of subphase pH (5.5 and 10.5) on the structure of behenic-acid monolayers was investigated during Mg2+ adsorption by means of Grazing Incidence X-ray Diffraction (GIXD) and Brewster Angle Microscopy (BAM). The final phase corresponding to an ion superlattice commensurate to the behenic-acid cell is pH-independent. In contrast, the sequence of phases evidenced from the initial L-2-phase to this final state presents at pH 10.5 an additional stage associated to a film condensation toward the L'(2)-phase. The structures of the intermediate states preceding the superstructure nucleation are slightly different, both with a short-range fatty-acid order. Finally, a laser light effect that could result from visible light absorption by the inorganic complexes is evidenced in the final state as well as in the intermediate phases.