The structure of the Mg-exchanged forms of three natural phillipsite samples with SU(Si+Al)=0.63, 0.72, and 0.76 arc described. In the more acidic samples, exchanged Mg concentration is lower than 2 a.f.u. and Mg is fully hosted in site II. In sample (1), Mg is 2.4 a.f.u. and occupies and intermediate position between II and II'. As a consequence residual K in site I migrates to the centre of the type I cage inducing a relaxation in the structure which changes from monoclinic P2(1)/m to orthorhombic B2mb. The structure solution was troublesome because some phillipsite crystals surprisingly did not exchange for Mg.