Abstract In the measurements of the time dependency of the surface tension (γ) by the maximum bubble pressure method, the effective age of the surface of the solution (T) is smaller than the measured interval between the separation of the two subsequent bubbles (t). The relation between T and t has been calculated for several examples of the dependency of γ on t. These calculations are based on the change in surface area (A) of a bubble formed in a surfactant solution on the tip of the capillary. From the results of these calculations the relation between γ and T was obtained.