Heat capacities and spontaneous heat evolution/absorption effects in solid solution systems between dibenzofuran and fluorene were measured with an adiabatic calorimeter. Glass transitions due to freezing-in of the reorientational motion of molecules in the stable crystalline state were observed throughout the composition range. The enthalpy relaxation processes associated with the transition were precisely tracked around 300 K by the temperature jump method under adiabatic conditions. The processes exhibited a non-exponential character and the non-exponentiality characterized by the parameter of a stretched exponential function was found to increase as the sample became impure with doping of the second component into the pure substance. The relaxation function was, meanwhile, found to show a linearity against the magnitude of the temperature jump. Both the non-exponential and the linear characters were interpreted on the basis of computer simulation experiments as being due to the presence of the distribution of activation energies for the reorientation of molecules.