Conducting polymers, such as polythiophenes, are promising low voltage actuator materials. The performance of these materials has improved significantly in recent years. A remaining problem, however, is the significant decline in the actuation strain produced when the external stress is increased. Using a poly(3-methylthiophene) actuator in an ionic liquid electrolyte it is shown that the isotonic actuation actually increases with increasing applied stress. The work output per cycle consequently increases at higher stresses, which is very desirable for practical devices. Although theoretically predicted, these results are the first demonstration of the increased actuator performance at higher operating stresses. The actuation behaviour was found to be strongly dependent upon the operating electrolyte and explained by the shifts in the elastic modulus of the actuator material that occurred during operation. (c) 2006 Elsevier Ltd. All rights reserved.