The use of perdeuteropolyethylpropylene (d-PEP) as a cryoprotectant for neutron studies of molecular organic solids is demonstrated by the solution of the structure of L-(--)-ephedrine hemihydrate from neutron powder diffraction data on a sample containing 20 wt% d-PEP. It is demonstrated that no contribution from d-PEP to the scattering is observed. Neutron diffraction studies of solvent-containing molecular crystals that require a perdeuterated cryoprotectant oil are thus possible.