Stabilisers consisting of metallic salts of p-butoxyphenylcyclohexyl phosphinic acid were introduced into two industrial copolymers based on bisphenol-A and phenolphthalein respectively, and their effects on the mechanical and thermal properties were investigated. The mechanical properties were sensitive to the amount of stabiliser. Ductility and breaking strength increased with small additions, but as the amount of stabiliser increased, the process was reversed. On the other hand the modulus of elasticity showed a steady decrease. Similarly, the thermal stability also increased with small concentrations of additive, and decreased at higher concentrations. Heat treatment increased the brittleness and reduced the breaking strength.