Clay minerals show various chemical compositions according to numerous cation substitutions occurring in their interlamellar space, tetrahedral and octahedral sheets. Chemical analyses are often performed at macroscopic scale and the deduced structural formulae do not necessarily reflect the actual crystal chemistry oi clays. The aim of this paper is to point out the powerful use of spectroscopic methods in the clays study, especially with regard to this problem. Even though the relationship between clay structure and IR spectrum is complex and has been only partially rationalized up to now, the spectrum reflects crystallochemical data on which other techniques cannot easily give information. The vibrations oi the OH groups are almost fully understood and have been found to be sensitive indicators oi the hydroxyl environment. The hydroxyls stretching frequencies are affected by the configuration of, and charge distribution in the surrounding octahedral cations to which the OH group is coordinated, and to a lesser extent, by tetrahedral charge of 2:1 clay minerals. While IR and NMR spectroscopies are shown to be good tools to investigate the cationic distribution in the octahedral sheet (thanks to OH bonds vibrations) and in the tetrahedral sheet of clay minerals respectively, the interlamellar space is well studied by far IR and Raman spectroscopies. After presenting the kind of data which can be obtained by the use of these spectroscopic methods, some solid solutions in phyllosilicates are studied using clay synthesis and a multi analytical approach. To measure the cation size influence on the characteristics of solid solutions, Si-x-Ge-1-x-talcs (0 less than or equal to x less than or equal to 1) were synthesized at 200 degrees C with Various octahedral cations (Ni2+, Co2+ or Mg2+) (MARTIN et al., 1992). XRD reveals preservation for Ge-talcs, of the lattice symmetry group of Si-talcs. An increase in the cell Volume and a rotation of tetrahedra (to adjust tetrahedral and octahedral sheet together) are observed as the Ge content of samples increases. The distribution of Ge and Si within the tetrahedral sheet oi the synthesized talcs were investigated by EXAFS and FTIR spectroscopies, and Si and Ge appear to be randomly distributed in the tetrahedral sheet (MARTIN ct al., 1995). in spite oi very different Si4+ and Ge4+ cation sizes, the Si-Ge solid-solution in talcs is complete and continuous, and the cation distribution is disordered. The influence of temperature on cation distribution was measured by studying series of trioctahedral synthetic 2:1 clays, Random distribution of octahedral cations is observed for samples crystallized at high temperature. When synthesis temperature decreases, a tendency to clustering is evident. The octahedral Fe-Mg and Al-Mg series in smectites were studied to exemplify heterovalent solid solutions (GRAUBY et al., 1993, 1994). Ail synthesized clays were tetrahedrally charged thus leading to solid solutions between beidellite, saponite, and nontronite end-members.