The effect on the vibrational spectrum of the hydroxy groups in dioctahedral 2:1 phyllosilicates of the isomorphous cation substitution of Mg(2+) by Al(3+) in the octahedral sheet was investigated at the DFT level. Ortho, meta and para Mg(2+) configurational polymorphs were defined. The theoretical vibration frequencies of OH groups depend significantly on the nature of the cations they are joined with. Theoretical values are spread out over narrow ranges: 3,612-3,626 cm(-1) for ν(AlOHMg), 3,604-3,606 cm(-1) for ν(AlOHAl), and 3,657-3,660 cm(-1) for ν(MgOHMg); 803-830 cm(-1) for δ(AlOHMg), 877 cm(-1) for δ(AlOHAl), and 693-711 cm(-1) for δ(MgOHMg), in agreement with known experimental values. From the intensities of the XOHY bands, we observe that the vibrational adsorptivities of the ν(OH) vibrations are not the same for all XOHY groups, and that ν(MgOHMg) absorptivity is much lower than that of ν(AlOHAl). These theoretical results should be taken into account in quantitative analysis of experimental vibrational studies in clay minerals, introducing different molar extinction coefficients in the Lambert-Beer law to determine the relative concentrations of both cationic arrangements.