The in vitro degradation of nanospheres perpared from three benzyl ester derivatives of poly (13-malic acid) containing 80, 90 and 100% of benzylated malic acid units was studied. The progressive decrease in the molecular weight of the copolymers was observed as the nanospheres degraded, demonstrating that the degradation under the experimental conditions occurred by a simple hydrolytic cleavage of the ester bond between the monomeric units. The degradation was slow, with the weight average molecular weight decreasing to about 70% of the initial value in 5 months for all the nanosphere systems. A comparison of degradation rates for benzyl ester copolymers with the degradation rate of poly (13-malic acid) homopolymer demonstrated a decreased degradation rate of benzylated copolymers which suggests that the introduction of a pendent benzyl ester function in proximity to the ester bond in the main chain, reduces the rate of the bond cleavage. No significant difference in the degradation behaviour of highly benzylated copolymers, containing 90 and 80% of benzylated malic acid units, and fully benzylated polymer could be detected to prove an autocatalytic role on degradation of the free pendent carboxyl group in the former two copolymers.