We have investigated the low-temperature heat capacities arising from the low-energy excitations in several molecular glasses. The heat capacities of glassy toluene, ethylbenzene, and 3-methylpentane were measured with an adiabatic calorimeter. The first two samples were doped with 10 mol % of benzene to prevent crystallization. The low-temperature heat capacities of the molecular glasses measured in this study and those measured previously were all reproduced well by the sum of a normal part represented by the Debye function and an excess one represented by the soft potential model. The peak energy of G(omega)/omega(2) [G(omega): density of states, omega: energy] was found to be proportional to M-1/2 (M: molecular mass:) for the hydrocarbon molecules but not for the alcohol molecules. Intermolecular hydrogen bonds in the alcohols may be responsible for the different behavior. (C) 1997 American Institute of Physics.