Plastic prepared from formulations of cornstarch and poly(beta-hydroxybutyrate-co-beta-hydroxyvalerate) (PHBV) biodegraded in tropical coastal waters. Biodegradation was monitored for 1 year. Starch-PHBV bioplastic appeared to lose weight in two overlapping phases until both biopolymers were entirely consumed. To examine the underlying degradation of starch and PHBV from biphasic weight-loss profiles, a semiempirical mathematical model was developed from which degradation profiles and lifetimes of the individual biopolymers could be predicted. The model predicted that starch and PHBV in the bioplastic had half-lives of 19 days and 158 days, respectively. Computed profiles also predicted that the starch in the composite would be completely degraded in 174 days, while residual PHBV would persist in the marine environment for 1107 days. The model further revealed that, for a 30% starch : 70% PHBV composite, PHBV degradation was delayed 46 days until more than 65% of the starch was consumed. This suggested that PHBV degradation was metabolically repressed by glucose derived from starch. Glucose repression of microbial PHBV degradation was substantiated in 91 of 100 environmental isolates. The validity of the elaborated model was proven when its revelations and predictions were later confirmed by chemical analysis of residual bioplastic samples. (C) 2000 John Wiley & Sons.