AbstractThe anharmonicity correction factors for H2Se and D2Se are estimated by the method of Dennison (1). The potential constants are calculated by means of three different methods; Wilson's F‐G matrix method (2), the kinetic constants method (3) and the L‐approximation method (4)‐using the presently evaluated harmonic frequencies. The potential constants thus obtained are utilised in evaluating the other molecular constants such as compliance constants, mean amplitudes of vibration, inertia defect, Coriolis coupling constants and centrifugal distortion constants. The values of inertia defect and centrifugal distortion constants obtained presently are in good agreement with the experimental values (5,6).