The novel system [acetic acid/phosphotungstic acid (H₃PW₁₂O₄₀)/octanol] was proposed for catalytic solvolysis of cellulose and for obtaining cellulose nanocrystals (CNC). Several alternative experiments involving mixtures with different compositions were carried out; reaction time was also varied. CNC particles from cotton, linen, softwood and hardwood cellulose were prepared in the experiments performed for 40 min in the presence of 0.25 mol% of the heteropolyacid. CNC samples were characterized by transmission electron microscopy and atomic force microscopy, X-ray diffraction analysis, and thermogravimetric analysis. It was established that the resulting nanoparticles had high crystallinity and rod-like shape; their length varied from 160 to 400 nm (cotton CNC had the shortest length, and linen CNC had the longest length), and CNC thickness ranged from 6 to 10 nm. Thermal stability of CNC was lower than that of initial celluloses and decreasing in the following sequence: cotton > softwood > hardwood > linen cellulose.