Measurements have been made of the forces required to peel a thin layer of a model viscoelastic adhesive off a rigid substrate. Over a wide range of temperature and rate of peel the results were found to yield a single master relation in terms of peel rate when reduced to a reference temperature by means of the Williams, Landel & Ferry rate-temperature equivalence for viscous materials. The relations obtained were complex, however, with two main features: a cohesive-adhesive failure transition at low rates and a sharp decrease in peel strength at high rates. The first effect is shown to be due to a change in the deformation process in the adhesive, from a liquid-like to a rubber-like response. An approximate relation between peel strength and the tensile stress-strain behaviour of the adhesive is developed in terms of a single empirically-determined parameter, the interfacial bond strength. Values of this parameter are deduced for several substrates. Measurements of peeling under hydrostatic pressure showed that cavitation of the adhesive did not occur in these experiments. The second effect is shown to be due to the transition from a rubber-like to a glass-like response of the adhesive. It is also shown to depend on the way in which separation is effected. Some general principles governing the mode of failure and the development of maximum peel strength are outlined.