The surface tension of an aqueous solution has long been recognized as one of the more important physical properties controlling its wetting and detergent powers, though early attempts to correlate these powers with equilibrium surface tension met o with limited success. It is becoming generally appreciated that factors other than solute concentration influence surface tension, and that the condition of the surface must also be taken into account. Thus, the surface age involved in many detergent processes does not exceed a few seconds, and much of this time may be required for the establishment of diffusion equilibrium. For dilute solutions of longchain compounds the operative dynamic tension may vary over a range of 40 dynes/cm. and acknowledgment of the time required for surface equilibrium is clearly essential. A further factor, the change in equilibrium surface tension which occurs during change in area of a soluble film, has not previously been considered, but may prove to have considerable significance.