The use of three-layer electroactives polymers is becoming more widely known among researchers and engineers because of their potentials to become mechanical actuators. The up-to-date research and development in the Nafion (R)/Metal composites has given some fundamentals, preparation techniques, and modeling of the phenomena regarding the deformation of the material from different scientific point of view. This gives the opportunity to propose correlations for modeling the phenomena present in these materials by different approaches. A constitutive model for the bending deformation of a Nafion (R) Ionic Polymer-Metal Composite (IPMC) was formulated based on an approach that represents, analogically, a phase transformation inside the base polymer and combines it with the bending of a beam in order to predict the displacement when the material is subject to an electrical input. The model was solved and evaluated for five different types of counter-ion systems. Experimental data was used to solve the parameters within the model. The results of the solved model gave a good fitting of the experimental data and are shown for different voltage and frequency conditions in all five ionic systems. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 3284-3293, 2009