The thermotropic mesomorphism of some cellulose derivatives (CD) based on the (2-hydroxypropyl)cellulose (HPC) was investigated. Three types of derivatives: two esters (PPC, HxPC) and cyanoethyl derivative (CEPC) were prepared. The X-ray diffraction patterns of CDs were compared with the differential scanning calorimetry, thermooptical, and mechanical measurements within a broad range of the temperature. Two relaxation processes alpha(a) and alpha(m), observed in the solid state of HPC, are also exhibited by all CDs, however, at lower temperatures. The alpha(m) relaxations, which indicate the transition from frozen anisotropic phase to mobile liquid crystalline (LC)phase, are shifted towards the lower temperatures with a corresponding increase in the d-spacing of the poly(saccharide) main chains of CDs (as seen in the X-ray measurements), The transition temperature to isotropic phase T-ni as well as glass transition temperature T-g (alpha(a)-relaxation) of the investigated CDs depend on the interactions between the lipcophilic side chains and the hydrophilic poly(saccharide) main chains of CDs. These interactions are determined by the length and polarity of the lipophilic side chains. The observed changes in the transition temperature to isotropic phase T-ni for CDs is consistent with the assumption that LC-organization of the poly(saccharide) main chains is stabilized by the lipophilic side-chains system. A significant increase in the length of the lipophilic side chains leads to nonlinear conformation, thus reducing the influence of van der Vaals forces, and consequently lowering T-ni. The polymer with high polarity lipophilic side chains (CEPC) exhibits higher T-ni in comparison to the ester derivative PPC with the same length of the side chains but having lower polarity. The stabilization effect of the lipophilic side-chains system on the LC-organization of the poly(saccharide) main chains is determined by the dynamic balance between length and polarity of the lipophilic side-chains system. (C) 2000 John Wiley & Sons, Inc.