The effect of mixing p-chloro-N-phenylphthalimide (p-Cl-NOPI) with the reference stabilizers: dibasic lead carbonate (DBLC), dibasic lead stearate (DBLS), barium-cadmium-zinc stearate (Ba-Cd-Zn stearate), n-octyltin mercaptide (n-OTM) or dibutyltin maleate (DBTM) on the stabilizing efficiency in thermally degradation of rigid PVC at 180 o C, in air, has been investigated. Mixing was effected in the range of 0-100 wt.% of p-Cl-NOPI relative to the reference stabilizers. The stabilizing efficiency is evaluated by measuring the length of the induction period (T s ), the period during which no detectable amounts of hydrogen chloride gas could be observed, and also from the rate of dehydrochlorination as measured by continuous potentiometric determination on one hand, and the extent of discoloration of the degraded polymer samples on the other. The results reveal that mixing of the stabilizers improves the Ts values, decreases the rate of dehydrochlorination and lowers the extent of discoloration of the polymer. This improvement attains its maximum when the p-Cl-NOPI is mixed with any of DBLC, DBLS, Ba-Cd-Zn stearate or DBTM reference stabilizers in weight ratio of 25/75%, respectively. Mixing of p-Cl-NOPI with n-OTM is an exception, as maximum synergism was attained at equivalent weight ratio (50/50%).