The dynamic surface tension of aqueous solutions of ethyl (hydroxyethyl) cellulose (EHEC) and hydrophobic modified ethyl (hydroxyethyl) cellulose (HM-EHEC) were determined using the maximum bubble pressure method. Values were monitored over surface lifetimes ranging from 0.15 to 2.5 s (after dead time corrections). In the low concentration range (<100 ppm) HM-EHEC was shown to be more surface active, and the presence of salt (0.1 M) was shown to increase the surface activity of both polymer systems. The results were compared with surface tension measurements carried out over longer time scales, determined using the du Nouy ring technique. From these results, isotherms were constructed relating surface tension to surface aging time. Although the results could not be directly correlated to interfacial diffusion models, the isotherms were found to be comparable to previous data reported for lower-molecular-weight poly(oxyethylene ether) surfactants and a higher-molecular-weight EHEC polymer determined using the pendant drop technique. Essentially, the isotherms could be divided into distinct regions : induction period, fast fall region where surface coverage occurs fairly rapidly, and finally the meso-equilibrium region. From the Gibbs equation, the number of segments of polymer per unit surface area at saturated adsorption levels was calculated over regions of different aging times, and the results are discussed in terms of configuration changes of the polymer at the interface.