Co-crystals of 4-methylpyridine and pentachlorophenol (MP-PCP) exhibit the very unusual property of isotopic polymorphism, formation of different phases by samples differing only in isotopic composition. Isotopically normal MP and PCP co-crystals are triclinic, but when the phenolic oxygen is deuterated, the structure is monoclinic. The triclinic deuterated and the isotopically normal monoclinic phases have not been described. Statistical thermodynamic modelling based on DFT calculations indicates that the triclinic deuterated form should become competitive at low temperature, and this has been confirmed by crystal growth at 243 K which yields a mixed monoclinic and triclinic sample. The aim of this proposal is to follow crystal growth of this form by in situ by neutron powder diffraction, confirming the calculations and obtaining unambiguously phase pure sample of the triclinic deuter