Silylation of cellulose and some low-molecular-weight hydroxy compounds (1,2-propanediol, D-glucose, methyl alpha-D-glucoside, sucrose and beta-cyclodextrin) has been performed with hexamethyldisilazane (HMDS) in liquid ammonia at elevated temperature in an autoclave. No salt-like by-products are formed and no tedious purification of the products is necessary. As indicated by size-exclusion chromatography the silylation of cellulose with HMDS/NH3 proceeds without degradation of the polymer chain. The degree of silylation of the trimethylsilylcellulose obtained is higher than with other silylation procedures reported in the literature. The temperature/composition diagram of HMDS/NH3 proved complete miscibility. above 24 degrees C.