The structures of natural intermediate tetraferriphlogopite from the Kovdor iron-ore deposit and tetraferribiotite from the Krivoi Rog Basin (both in the Ukrainian Shield) have been refined. A three-dimensional set of intensities has been obtained using an automatic single-crystal diffractometer. The structure of intermediate tetraferriphlogopite has been refined to R 0.059, from 1059 observed reflections, and that of tetraferribiotite to R 0.079, from 534 reflections. The average bond lengths in intermediate tetraferriphlogopite are: T-O 0.1665, M₁-O,OH = M₂- O,OH 0.2085, and inner K-O 0.2973 nm; the angle of rotation of the tetrahedra alpha = 9.24o. In tetraferribiotite, T-O 0.1679, M₁-O,OH 0.2111, M₂-O,OH 0.2098, K-O 0.2974 nm and alpha 8.96o. In both structures, the OH group is situated along the c* axis; the bond length in the two minerals is 0.0792 and 0.0887 nm. The pattern of distortions in the tetraferribiotite structure is analogous to that identified in phlogopite.-D.A.B.