Relevance. Liposomes have been known to science since the 1960s and remain relevant as delivery systems for active pharmaceutical ingredients (APIs). Liposomal delivery systems are actively used in the technology of dosage forms with antibiotics and hormonal agents; however, antiviral drugs also require special delivery systems capable of improving their bioavailability. One of the parameters determining the quality of liposomes is the encapsulation efficiency (EE), which depends on many factors, including the composition of the lipid phase. Comparing lecithins of different origins is of interest as a preliminary step in selecting the lipid base. Aim of the study. To obtain laboratory samples of a liposomal form of riamilovir and to determine the encapsulation efficiency of riamilovir depending on the lecithin used. Materials and methods. Liposomes were prepared by the hydration of a lipid film followed by ultrasonic treatment. Soybean and sunflower lecithins were used as the lipid base; the film was hydrated with an aqueous-alcoholic solution of riamilovir at a concentration of 10 mmol/L. After centrifugation, free riamilovir in the supernatant was determined by UV spectrophotometry at 365 nm using a calibration curve. The fatty acid composition of lecithin samples was analyzed by gas chromatography with flame ionization detection after preparing fatty acid methyl esters. Spectrophotometric measurements were performed in three analytical replicates. Results and discussion. In the experiment, samples of the liposomal form of riamilovir based on soybean and sunflower lecithin were obtained. The encapsulation efficiency of the API into liposomes prepared from soybean lecithin was 59,94%, and from sunflower lecithin – 82,28%. To determine the possible cause of these differences, chromatographic analysis of the lecithin samples was performed. During identification of volatile fatty acid methyl esters present in different lecithins, it was found that soybean lecithin, unlike sunflower lecithin, contains 6.6% of unsaturated linolenic acid. The obtained EE value is consistent with passive encapsulation of hydrophilic compounds; however, quantitative conclusions are limited by the calibration (R² = 0.9478). To improve method reliability, expansion of concentration levels and evaluation of validation characteristics are required. Centrifugation in its current form may not guarantee complete separation of nanosized vesicles, necessitating the use of dialysis or ultrafiltration. The revealed differences in the fatty acid profiles of lecithins, particularly the presence of linolenic acid in the soybean sample, may explain the decrease in EE due to increased membrane fluidity. Conclusion. The encapsulation efficiency of the API into liposomes prepared from sunflower lecithin exceeds the corresponding value for liposomes from soybean lecithin by 22.3%. One of the reasons for the low encapsulation efficiency may be leakage – the escape of the active substance from the vesicle lumen – due to differences in the membrane fluidity of the liposomes.