Catalytic hydrodechlorination (HDCl) of 2,3,4',6-tetrachlorobiphenyl and 2,3,3,5-tetrachlorobiphenyl has been studied in the presence of a sulphided Ni-Mo/gamma-Al2O3 catalyst. The reaction runs were carried out at constant temperature and pressure in a stirred batch reactor using hexadecane as the reaction medium. Temperature levels of T=250 degrees C and T=300 degrees C at a hydrogen pressure of P-H2 = 20 bar were tested. The results demonstrate that the HDCl process proceeds via an irreversible stepwise pattern. The kinetic constants of all observed intermediate HDCl steps were evaluated. The selectivity of HDCl reactions has been interpreted on the basis of electrophilic aromatic substitution mechanism. It shows that the HDCl rate is slower when carbon atoms adjacent to a substituted chlorine atom are bonded to other chlorine atoms. This depends on the inductive effect of chlorine atoms. It also shows that ortho-substitution is slower than meta- and para-substitution. In this case, both steric and electronic effects are probably relevant.