The enthalpies of evaporation, the saturated vapor pressures of 1-methylcyclopentanol (I) and 1-chloro-1-methylcyclopentane (II) and the energy of combustion of crystal I were measured. IR and Raman spectra of both compounds were recorded and vibrational analyses were made. Thermodynamic functions of I and II in the ideal gas state were calculated by the statistical thermodynamics methods. In order that the experimental and calculated entropies of gaseous 1,1-CIMeCP agree, it is necessary to determine the contribution of pseudorotation as the contribution of motion with potential function V(phi) = {5800 - 3060 cos(phi) - 2740 cos(2 phi)} J mol(-1) and pseudorotational moment of inertia 15.24 x 10(-40) g cm(2). The molecular rotation in the plastic crystal state of II is discussed.