Direct fluorination of acetyl- and ethylcelluloses was performed in a bubble reactor with liquid perfluorinated medium (perfluorodecalin). Concentrated fluorination mixtures (F ₂ + N ₂) and long reaction times (up to 3 h) were used for better control of the degree of fluorination than it was made previously in direct gaseous fluorination of polymers. The fluorine consumption in the runs with both polymers only slightly increased with increasing temperature and fluorine concentration in the gas mixture. Fluorine content in the fluorinated polymers did not depend strongly on the fluorination conditions. X-ray analysis, differential scanning calorimetry of the fluorinated samples showed no significant changes in respect of the initial polymers. However, emergence of novel and splitting of initial absorption bands, the change of polarization of C–O bonds in IR spectra for the fluorinated samples were observed. It can directly and indirectly justify the appearance of the C–F bonds in the side groups and glycopyranosyl ring of the fluorinated substituted celluloses. ¹⁹F, ¹³C solid NMR spectroscopy of the fluorinated samples confirm hydrogen-substituted fluorination of methyl groups.