We present the results of the research into the Xe2-excilamp radiation action (λ = 172 nm) on the phase transitions in natural gas containing water vapor in the temperature range from - 30 up to + 22°С at a gas pressure of 22.2 bar in the reactor with an active volume of 5000 cm3. The Xe2-excilamp radiation power was 4.4 W; exposure time was 1 min. Determination of the gas mixture composition before and after irradiation was performed by means of gas- and liquid chromatography. It has been shown that irradiation causes a twofold decrease in water vapor concentration and a considerable increase in hydrocarbon content (alcohols and dimers) with a number of carbon atoms greater than 6. After irradiation, the temperature dependence of natural gas pressure was measured within the range from -30 up to +22°С and was compared with temperature dependence of unirradiated gas pressure within the same temperature range. The comparison showed two representative parts of the dependence, which demonstrate that products formed in the reactor gave a considerable increase of gas pressure (about 0.4 bar) in the temperature range from - 30 to - 15°С.