Eight samples of trimethylsilyl cellulose (TMSC) with different degrees of substitution (DS) were investigated by static and dynamic light scattering in tetrahydrofuran at 25 degreesC. Six of these sample were prepared from Avicel cellulose (DPw = 221) by controlled silylation with stoichiometric amounts of hexamethyldisilazane in liquid ammonia at elevated temperature in an autoclave. The two others were obtained by controlled desilylation of the sample with DS = 2.72. The TMSC samples were soluble in tetrahydrofuran, but in all cases strong aggregation was observed with aggregation numbers of 80 for DS = 1.96 up to 270 for DS = 2.72. Surprisingly, the aggregation number considerably increased for the samples obtained by desilylation. These very large aggregates permitted a detailed analysis of the angular dependencies which resulted in rather rigid structures with Kuhn segment lengths of l(K) = 122-174 nm, about 6 times the segment length of molecularly dissolved cellulose derivatives. This increase in chain stiffness is caused by a side-by-side alignment of 2-4 chains, which further increased to about 13-14 chains for the samples obtained by partial desilylation. The structure of the aggregates resembles that of branched macromolecules built up from stiff chains in which the junction zones of 2-4 aligned chains act as branching points.