Slow strain rate tensile tests were conducted on un-notched PMMA samples in a range of liquid environments in order to assess the effects of diffusion on craze formation. The environments were selected to have a spread of diffusion rates and consisted of water, methanol, propan-2-ol, 355 trimethylhexanol and ethylene glycol. Absorption tests in these environments showed that methanol had the fastest diffusion rate and greatest degree of swelling, followed by water. Ethylene glycol and trimethylhexanol showed no weight gain, and propanol caused a steady dissolution of the PMMA. Tensile tests were conducted after the samples had been pre-immersed in the liquid environments for times ranging from 1 minute to 14 days. These were compared with equivalent tests in air, with the point at which the environment curve departed from the air curve being used to establish where crazing commenced. Tests were also conducted in methanol and ethylene glycol at a range of strain rates. It was found that with methanol and to a lesser degree with water and trimethylhexanol, crazing was delayed in tests with longer pre-immersion. This is due to mechanisms of flaw blunting and/or the introduction of surface compressive stresses. Similar effects were seen with the methanol tests at slower testing rates. It was found that these effects can occur very locally to the sample surface, where no weight gain could be measured and in the very early stages of craze development.