Illites from the volcanogenic Zn-Au-Ag sulfide deposit Galkinskoe (Northern Urals) have been investigated using oblique-texture electron diffraction patterns in combination with the simulation of diffraction effects. Along with single-phase tv/ cv1 M illites, the structures of which are formed by statistically interstratified dioctahedral 2: 1 layers with vacant trans- or cis-octahedra, illites in the form of a mixture of two interstratified tv/ cv1 M phases, with the dominance of tv- or cv layers, are identified. It is shown that in the latter case the diffraction pattern can also be interpreted within the single-phase tv/ cv model, provided that the tv- and cv layers are distributed with some tendency to segregate. The structural features indicating the preference of the two-phase model are analyzed.