The isotope effect has been measured in acetylenedicarboxylic acid dihydrate. Comparisons have been made with oxalic acid dihydrate. Expansion of the crystals occurs on substituting deuterium for hydrogen. The relative magnitude of the effects corresponds with the relative strengths of the acids on the hypothesis that the short isotope bonds involve an overlap of two structures. Brief reference is made to a new thermal transition in the acetylene acid, and to an unexplained split reflexion in the deuterium compound.