The hybrid organic-inorganic perovskites are of great interest as functional materials, because they combine the well-established perovskite topology with new physics and chemistry that arise from including organic components. In particular, hydrogen bonding plays an important role in their structure and hence functionality; thus isotopic substitution can make a big difference to their behaviour. Our target material here, DMACo(HCO2)3, is an extreme example: the deuterated compound displays a phase transition on heating that is not observed in the protiated analogue. We propose a study of the differences between the structures of these isotopologues by high-resolution neutron diffraction. Our results will help to understand this material’s multiferroic behaviour, but also more generally to reveal the ways in which isotopic substitution can be used to tune this family’s properties.