The crystal structure of kassite, ideally Ca[Ti2O4(OH)2], containing 2 wt% Cr2O3, from the Saranovskoye chromite deposit, Perm’ district, Northern Urals, has been determined and refined to R1 = 0.06 using single crystal X-ray diffraction data. The crystals have monoclinic symmetry, P21/a, with a = 5.275(1), b = 9.009 (2), c = 9.557 (2) Å, β = 90.43°. A pronounced sub-structure for the mineral, conforming to space group I2/a, is related to the I2/a structure for lucasite-(Ce), Ce[Ti2O5(OH)]. It comprises (001) layers of gibbsite-like fused hexagonal rings of edge-shared Ti(O,OH)6 octahedra with the Ca atoms sandwiched between pairs of opposing rings and displaced from the center of the rings along . Ordering of the protons in chromian kassite lowers the symmetry to P21/a. Kassite, CaTi2O4(OH)2, and cafetite, CaTi2O5(H2O), are identical chemically but significantly different in their crystal structures.