We measured the heat capacities of methyl-monodeuterated lithium acetate dihydrate single crystals (LiC2H2DO2 center dot 2H(2)O) at 2 K < T < 20 K and Raman spectra at ambient conditions. Significant excess heat capacities were found at T = 14.5 K and 5.5 K. The thermodynamic data is compared to that of fully protonated and fully deuterated LiAc and interpreted in terms of phase transitions between a classical dynamically disordered phase above 14.5 K, a phase stable between 14.5 K and 5.5 K in which there is quantum tunnelling of the methyl groups, and an isotopically ordered phase below 5.5 K. (C) 2012 Elsevier Ltd. All rights reserved.