The effect of Tween 20 on the penetration of ketoprofen in aqueous vehicles through excised rat skin has been evaluated. The addition of Tween 20, up to 2%, in water containing ketoprofen decreased the penetration of ketoprofen through the skin compared with the reference vehicle without this surfactant. However, the addition of Tween 20 in ketoprofen aqueous vehicle increased diffusion parameter significantly which was compensated with the lowered partition parameter.