Heat capacities of dibenzofuran and fluorene were measured with an adiabatic calorimeter in the temperature ranges 245 < (T/K) < 375 and 200 < (T/K) < 419, respectively. Glass transitions were discovered at T-g = 316 K for both the compounds in the stable crystalline state due to freezing-in of the reorientational motion of the pseudo-planar molecule. In dibenzofuran, the molar-heat-capacity jump at T-g was estimated to be (6.8 +/- 0.2) J . K-1. mol(-1) and the molar activation energy for the reorientation to be (98 +/- 5) kJ . mol(-1). In fluorene, the molar-heat-capacity jump at T-g was estimated to be (7.0 +/- 0.3) J . K-1. mol-1, and the enthalpy relaxation as a function of time showed a remarkable non-exponentiality. The latter was interpreted potentially to be related to non-planarity of the molecule.