Values of the inner-sphere reorganization energies (REs) for diatomic molecular redox couples in gaseous electron transfer (ET) processes have been calculated on the basis of the improved self-exchange model of the reorganization phenomenon and accurate potential functions at the reorganized state from ab initio calculations at the Hartree-Fock self-consistent field level (HFSCF 6-31G*, 6-31GE, DZ, and DZP(d)). The structural reorganization index (activated radius of the reduced molecule) is given. The inner-sphere RE values were calibrated by comparing molecular structural parameters (bond lengths) with experimental data. Results indicate that the ab initio calculation gives values in better agreement with experimental ones for the inner-sphere RE and is an accurate and reliable method to analyze the structural reorganization effect of reactant molecules in gaseous ET processes. The errors caused for inner-sphere RE are within +/-500 cm(-1) (+/-1.5 kcal/mol) and are due to the restrictions of the HFSCF limit and basis set effect. It should be emphasized that, for gaseous ET processes, it is very essential to calculate accurately the inner-sphere RE, as the outer-sphere effect may be neglected and the inner-sphere reorganization effect of reactant is the main decisive factor which remarkably influences the ET rate by means of the Eyring equation.