Reported Values of Structural parameters, vibrational fundamentals, and potential energy functions for internal rotation of n‐butane and isobutane are reviewed. The selected values were used to calculate the thermodynamic properties (C°p, S°, (H°‐H°0)/T) in the temperature range of O to model. Contributions of internal rotation were evaluated by the direct sum of terms containing energy levels which were calculated with a one‐dimensional potential model. For internal rotation about the central C‐C bond in n‐butane, energy levels were approximated by two procedures. A inique potential function was assumed for each methyl rotor of n‐butane or of iso