AbstractThe structure of monoclinic Se2O5, S. G. P21/c, Z = 4, was solved by direct methods and refined by an anisotropic full matrix least‐squares to R = 0.116 for 855 densitometer intensities. The structure consists of zig‐zag chaines [Se(O)OSe(O)2O]nwith alternating Se(IV) and Se(VI) atoms. Each Se atom is coordinated tetrahedrally, Se(VI) by 4 O atoms, Se(IV) by 3 O atoms and a lone