The study focuses on determination of a set of crucial parameters for molar mass calculation of cellulose from the results of size exclusion chromatography coupled with multiple angle laser light scattering (MALLS) and differential refractive index (DRI) detectors. In the present work, cellulose has been derivatised to obtain cellulose tricarbanilate (CTC) soluble in tetrahydrofuran (THF). The parameters of Rayleigh scattering in the MALLS detector: refractive index increment (dn/dc) and second virial coefficient (A2) of CTC in THF were determined for laser wavelength 658 nm. In order to avoid errors resulting from cellulose derivatisation by-products present in the CTC solution, the so called “on-line” method of measuring dn/dc and A2 was applied. Based on the A2 determination, its influence on cellulose molar mass calculations and cellulose molecular dimensions were critically assessed. The latter includes evaluation of artificially aged cellulose towards conceivable branching by conformation plot analysis.