We have investigated competitive adsorption between homopolymer chains of different lengths. Optical reflectometry was used to monitor adsorbed amounts as a function of time, and an impinging jet flow cell was used to control mass-transfer rates between the solution and the adsorbing surface. For poly(ethylene oxide) (PEO) adsorbing from water onto silica we found that exchange of short adsorbed polymers against longer ones in solution takes place at constant coverage, the total adsorbed amount being equal to the saturated level of the short component. This result agrees fully with predictions for an adsorbed layer of mixed composition which is in full equilibrium with its immediate surroundings. We infer from this that relaxation times of adsorbed layers of PEO are shorter than the time scale of the experiment (seconds). For polystyrene (PS) adsorbing from decalin onto silica we also observed exchange, but for this case the total adsorbed amount increased during exchange, indicating incomplete relaxation of the polymer layer. It was found that the excess adsorbed amount with respect to equilibrium depends on the history of the adsorbed layer and on the rate of arrival at the surface of displacing long chains. Moreover, desorption of this excess adsorbed amount could actually be observed when the supply of polymer from solution was interrupted. From the latter type of experiments we determined a relaxation time of 45 s for pure decalin, but faster relaxation occurred in toluene/decaline mixtures and in tetrachloromethane. We forward arguments to show that the relaxation in these mixed layers is due to spreading of the adsorbed long chains from a highly coiled to a flat conformation.