The interaction of the anionic surfactant, sodium dodecylsulfate (SDS), with the hydrophilic surface of a thin cellulose film and the role of electrolyte (0.1 M NaCl) and the polyelectrolyte, poly(dimethyldiallyl ammonium chloride) [polydmdaac], have been studied by neutron reflectivity (NR). The thin cellulose films were prepared by Langmuir-Blodgett (LB) deposition of trimethylsilyl-cellulose (TMSC) on silicon, and the hydrophilic surface was produced by the cleaving of the terminal methyl groups of the TMSC by HCl vapor. Despite both the surfactant and cellulose surfaces being nominally anionic, SDS adsorption and swelling of the cellulose film occurred during adsorption. The results show that the nature of the adsorption and the extent of the penetration into the cellulose film can be controlled by the addition of electrolyte, NaCl, and cationic polyelectrolyte, polydmdaac.