Summary: The crystal structure of buttgenbachite from Likasi (Congo) was solved by a three- dimensional Patterson function, computed using intensity data collected photographically by the Weissenberg method, and refined by successive Fourier maps and least-squares refinement to an R index 0·055 for 328 independent observed reflections. The cell content resulting from the structural analysis is Cu_36·6Cl_6·7(NO₃)_2·6(OH)_63·9.2·1 H₂O. The space group is P6₃/mmc with a = 15·750 Å and c = 9·161Å. The buttgenbachite structure is not completely ordered. It consists of a rigid three-dimensional skeleton formed by Cu co-ordination polyhedra sharing edges and corners. The large channels in the framework show a zeolite-like nature; inside the channels the disorder is caused by the different orientations of nitrate groups and by the occurrence of the substitution 2NO3−⇄Cl−. The crystal structure of buttgenbachite is closely related to that of connellite. The mechanism of forming an isomorphous series is discussed: the present study shows that the replacement is more complicated than was supposed in the literature.