This paper focuses on the effects of temperature, humidity, and dimensions on the displacement of ionomer-based polymer actuators. The amount of displacement and velocities of the actuation strongly increased with increasing humidity and temperature. We attributed this behavior to a change in the Young's modulus (the stiffness) and ion conductivity based on water uptake. To evaluate the dependence of the velocity of the displacement on humidity and temperature, we examined three velocities (i.e. the initial, bending, and backtracking velocities). The observed increase in the bending velocity at higher relative humidity (RH) levels arises from an increase in the water uptake, which enhances ion conductivity and decreases the film stiffness. The ratio of the bending velocity to the backtracking velocity at higher RH decreased because of a drastic increase of the backtracking velocity at higher RH. This result would be explained by an increase in the ion conductivity accompanying a decrease in the stiffness. Furthermore, we compared the difference in the amount of displacement of the actuator using actuators of two widths (2 and 10mm) at 30, 60, and 90% RH and at 25 degrees C. The difference in the width of the actuator did not completely affect the displacement. These results are reasonably explained by considering the amount of mobile ions per unit volume of the films of the actuators. Copyright (c) 2016 John Wiley & Sons, Ltd.