Brownian motion (BM) of glass microspheres (similar to 1 mu m size) at the water-air interface in the presence of dodecylamine (DDA) is directly measured with the use of an image analyzing method. This method has been tested by BM measurements in the bulk and proved to be accurate. The experimental results, recalculated as particle diffusion coefficients D-p, are investigated from the view point of a theory earlier developed by us. According to this theory, D-p should be a function of the Marangoni number M alpha = E alpha/D mu, where E = (-Gamma partial derivative gamma/partial derivative Gamma) is the Gibbs elasticity, alpha = (partial derivative Gamma/partial derivative c) is the distribution coefficient, Gamma and c are the surfactant (DDA) adsorption and bulk concentration, and D is the surfactant bulk diffusion coefficient. Rather unexpected, D-p shows no significant dependence on Marangoni number even for almost saturated states of adsorption. More over, the values of the diffusion coefficients at the surface (D-p similar to 0.3 mu m(2) s(-1)) and in the bulk (D-p similar to 0.2 mu m(2) s(-1)) seem to be of one and the same order of magnitude. The discrepancies between the theory and experiment are discussed from a more general dynamic and adsorption state point of view.