A non-linear constitutive model was formulated for ionic electroactive polymers (EAP) to describe the energetic coupling between electrical and mechanical domains. The polymer was modeled as a multi-port energy storage element with inputs from the electrical and mechanical domain. Using energy conservation methods, general relationships between stress, strain, voltage, and charge were determined. A solution to the uniaxial loading boundary condition was developed fully and compared to a linear model published by Madden and an electrochemical model published by Mazzoldi et al. Experimental results from a conducting polymer actuator composed of polypyrrole were used to validate the electro-mechanical coupling model. It was found that the correlation between the model and experimental data was very good for strains up to 3% and applied voltages up to 1 Volt; these are within the typical operating range of polypyrrole. The model is sufficiently simply to allow real-time control while also exceeding the linear coupling models in its ability to predict polymer behavior in normal operating ranges.