The crystal and magnetic structures of two substituted variants of the multiferroic phase BiFeO3 will be studied as a function of pressure. Both Bi0.8La0.2FeO3 and Bi0.8Tb0.2FeO3 are orthorhombic (PbZrO3-structure type) and (GdFeO3-structure type) and antiferromagnetically ordered (commensurate G-types spin structures) at RT. It is hoped that through the application of pressure a transition to the (ferroelectric) R3c symmetry of BiFeO3 will occur. The Paris-Edinburgh cell will be mounted on WISH for the first time as its high resolution at high d-spacing is needed to resolve the expected splitting of the main magnetic intensity as the spin structure becomes incommensurate at the transition. Further, novel magnetoelectric coupling effects may be discovered if a R3c phase can be recovered.