Bacterial cellulose (BC) gels produced by Acetobacter xylinum were used as model substrates for studying xylan assembly. The assembly of xylan onto BC surfaces was carried out by exposing BC films to xylan solutions under autoclave treatments. The BC surfaces were found to retain significantly higher amounts of xylan than cellulose fibers from cotton or dissolving pulp under the same conditions. The morphology of BC/xylan surfaces was visualized using atomic force microscopy. Globular xylan particles in the nanometer scale were found on the cellulose surfaces even after short treatment times, suggesting that the surface structures are formed by adsorption of preformed xylan aggregates from the solution rather than being built up at the cellulose surfaces with time by multilayer adsorption of xylan. Dynamic light scattering measurements of solutions support this hypothesis by revealing increases in both size and amount of xylan aggregates with increased treatment time. The increased aggregation can be explained by the decreased amounts of solubilizing 4-O-methyl glucuronic acid substituents in the autoclaved xylans, as shown by carbohydrate analysis. The retention of xylans on cellulose surfaces under conditions similar to those in wood pulping is likely to take place through an adsorption process involving diffusion to, and subsequent interaction with, cellulose surfaces of preformed xylan aggregate structures.