Active polymer gels expand and contract in response to certain environmental stimuli, such as the application of an electric field or a change in the pH level of the surroundings. This ability to achieve large, reversible deformations with no external mechanical loading has generated much interest in the use of these gels as biomimetic actuators and ‘artificial muscles’. In previous work, a thermodynamically consistent finite–elastic constitutive model has been developed to describe the mechanical and actuation behaviours of active polymer gels. The mechanical properties were characterized by a free–energy function, and the model uses an evolving internal variable to describe the actuation state. In this work, an evolution law for the internal variable is determined from free actuation experiments on a poly(vinyl alcohol)–poly(acrylic acid) (PVA–PAA) gel. The complete finite–elastic/evolution law constitutive model is then used to predict the response of the PVA–PAA gel to isotonic and isometric loading and actuation. The model is shown to give relatively good agreement with experimental results.