Degradation of polymers caused by water ingress is a recognised problem for the long term strength of structural materials but may be considered too slow for some packaging materials! Following an earlier study of water degradation in an epoxy resin, we here consider effects in cellulose diacetate-a biodegradable material. As before, initial weight increase following exposure to water is followed by weight decrease such that, at sufficiently long times, there is a net weight loss. The overall behaviour has been wholly or partially observed at six temperatures ranging from 10 degrees C to 90 degrees C. A model based conceptually on the initial attachment of water to the polymeric structure (causing weight increase) followed by chain segment separation and subsequent leaching (leading to weight decrease) was developed and explains satisfactorily the overall behaviour.