Over the last twenty years, illite-smectite mixed layers (I-S), especially ordered ones, have been studied extensively in diagenetic series. Basically, I-S ate considered to represent stacks of smectite, presumably low-charge montmorillonite, and illite layers of fixed composition (about K-0,K-90 (Si-3,Si-30 Al-0,Al-70) (Al-1,Al-80 Fe-0,Fe-05 Mg-0,Mg-15) O-10(OH)(2)). With the increasing depth in burial, the amount of I-S crystals decreases for a given volume of sediments and the composition of I-S crystals becomes more illitic. Illite and I-S of varied compositions coexist in the same sample. With increasing burial depth, the dimensions of I-S and illite crystals increase both in the plane of the crystallographic a and b directions and along the c* axis (coherent scattering domains). Several models for I-S crystal structure have been proposed, deduced mainly from X-ray diffraction data and from the observations in transmission electron microscopy. These models are not consistent with what is known about possible growth mechanisms for I-S and illite. The model proposed allows us to describe the ordered I-S stackings and is consistent with these growth mechanisms. As I-S crystallites are composed of montmorillonite and of fixed composition illite, this model supposes the montmorillonite layers to be included within the I-S crystals, whereas external layers act as high-charge beidellite.