We have studied the time dependence of the interfacial tension between acidic aqueous solutions (pH 2) and commercial straight-run bitumen droplets of different origins by the pendent drop method. Because of the existence in bitumen of naturally occurring surface-active species, significant changes of the interfacial tension are observed over timescales of several thousands of seconds. The kinetics of the decrease is consistent with the exodiffusion of endogenous surfactants toward the bitumen-water interface and throughout the bitumen matrix. At T = 90 degrees C, one observes that the interfacial tension decreases as t(1/2) at short times and as t(-1/2) at long times when the interface becomes nearly saturated. Combining these two kinetic data allows for a direct estimate of the surface concentration of these endogenous surfactants. If one makes the assumption that they organize in a dense, close-packed monolayer, their size and concentration in bulk bitumen can also be readily derived. At T = 140 degrees C, the viscosity of bitumen is 20 times lower than at 90 degrees C and the short time kinetics can no longer be resolved with our experimental apparatus. This restricts the data analysis, but we can nevertheless obtain an estimate of the size of the surfactant molecules if one makes the reasonable hypothesis that the bulk concentration of surfactants remains unchanged between 90 and 140 degrees C. Our results suggest that the surface-active species at pH 2 are asphaltene moieties of basic character. They are probably in an aggregated form because the measured, nanometric size decreases at elevated temperatures. Quite remarkably, they correspond to a very small fraction of the total asphaltenes contained in bitumen, which explains why they are extremely difficult to detect analytically. To the best of our knowledge, this is the first time that quantitative data on the endogenous surfactants are obtained in industrial bitumen. The present results thus complete the numerous existing observations on model oil-water systems.