In this paper, the pH value of working solution of Ionic Polymer Metal Composite (IPMC) actuators was systematically changed and the effect of pH on the deformation behavior was experimentally investigated. IPMC actuators, which consist of a thin perfuorinated ionomer membrane and electrodes plated on both surfaces, can undergo a large bending motion when a small electric field is applied across its thickness direction. Because of its lightness, softness and usableness in wet conditions, IPMC actuators are promised to be used for artificial muscles, biomimetic actuators and medical applications. The deformation properties of IPMC actuators are influenced by working solutions. However, the basic understandings about the effect of pH value of working solution on the deformation properties have not been clarified yet. Therefore, the pH characteristics of IPMC actuator were evaluated in this paper. IPMC actuators with the palladium electrodes were used and the responses for step voltage in several pH solutions were investigated. The results showed that the deformation behavior is drastically changed between acid and alkali solutions. In acid solutions, IPMC actuator showed a relaxation motion, though IPMC actuator in alkali solutions kept its deformed shape during applying a voltage.