Abstract In order to determine the dihedral angle of the gauche isomer of CH 2ClCOCl, normal frequencies and band intensities have been calculated at various assumed rotational angles of the gauche form. It has been shown by these calculations that the most probable value of the dihedral angle is about 105°. By applying the “absolute intensity method” and using the observed intensities of CH 2C1COC1 and CH 2BrCOBr measured in solution, the energy differences between, the rotational isomers of these molecules have been evaluated. The “direct intensity ratio method” has also been proposed as a simplified method to obtain the energy difference, and its applicability has been discussed. The energy differences of CH 2ClCOCl, CH 2BrCOBr and CHCl 2COCl determined by these two methods have been compared with those determined previously by the temperature variation method.