The effect of solvent on the kinetics of the alkaline depolymerisation of poly(ethylene terephthalate) (PET) has been investigated. The process was found to be first-order with respect to hydroxide ion concentration, and the activation energy for the reaction in water was found to be 69 +/- 13 kJ mol(-1) (95% confidence limits). This is higher than the reported activation energy in the homogeneous molten state depolymerisation, but lower than the reported activation energies for the neutral solid-state heterogeneous hydrolysis. The choice of solvent plays an important role in the alkaline depolymerisation process; strong solvent-polymer interactions can facilitate the depolymerisation by causing swelling to mitigate diffusion constraints, and strong solvent-solvent interaction can enhance the nucleophilicity of the hydroxide ion. However, the first-order kinetic behaviour with respect to [NaOH] has been shown to be applicable over a limited range in some solvent systems, the limitation being due to ion-pairing which leads to reduced activity of the hydroxide ion.