silicon-29 and 'h nmr spectroscopy were used to investigate the hydrolysis of methyl silicones in a variety of aqueous environments. dilute acid, dilute base and one or more of the dissolved constituents of blood plasma were found to catalyse the degradation of polydimethylsiloxane (pdms)-and its hydrojty- and methoxyterminated derivatives-along with the interior and shell of a silicone mammary prosthesis. dimethylsilanediol (dmsd) was the principal hydrolysis product, although in many instances (most notably after long decomposition periods) the dimer (tetramethyldisiloxanediol) and/or certain cyclodimethylsiloxane species were also detected. only for hexamethyldisiloxane was silicic acid detected as an additional hydrolysis product after long-term interaction with blood plasma. alcohols and certain aliphatic polyols, when added to solution, typically caused alkoxylation of dmsd and the dimer. the presence of electron withdrawing hydroxy and methoxy end groups greatly increased the rate of degradation of pdms. both the shell and interior of the mammary prosthesis also hydrolysed faster than pure pdms, which would suggest that there is considerably less than 100 % trimethylsilyl capping of their constituent polymer chains.