Kinoite, Cu2Ca2Si3O10·2H2O, has space group P21/m with cell parameters a = 6.991, b = 12.884, c = 5.655 Å, and β = 96°11' with Z = 2. The structure has been determined using automated diffractometer data. Refinement converged with R = 0.037 for all observed reflections. The structure consists of ladderlike (CuO2)n chains parallel the c axis, Si3O10 multiple tetrahedra units, and distorted calcium-oxygen octahedra. Alternate pairs of oxygens in the CuOa chains are water oxygens that also serve as apices of the calcium octahedra.