The Ga for A1 substitution in kaolinites and smectites was studied using clay synthesis from initial gels having an (Al + Ga)/Si ratio of kaolinites and MGa = Ga/(Ga + A1) ratio ranging from 0 to 1. Only kaolinite was obtained for MGa in the range 0-0.10. For higher MGa, synthesized clays were both kaolinite and beidellite, or pure beidellites. The evolution of b cell parameters, and the appearance of a vAI-OH-Ga absorption band at 3600 cm-1 in FFIR spectra, proves the progressive substitution of AI by Ga in kaolinite. The low substitution of AI by Ga in kaolinite can be correlated with the large difference in ionic radii between Al3+ and Ga3+ which causes a large increase in cell dimensions. By contrast, the incorporation of Ga in smectites is easy and can be complete.