[en] Studies of the behaviour of straw proportional tubes, 4 mm in diameter, under sustained irradiation were carried out with a Xe-CO₂-CF₄ [50-30-20] gas mixture. No observable gain drop has been found to affect the tubes up to reasonable doses (≤ 1 C/cm of wire). The chemical modification of the gas composition, causing the formation of electronegative, relatively stable species during the avalanche processes in a highly irradiated straw tube, is suggested to be responsible for a decrease of the pulse height in a second proportional counter (12.5 mm drift distance) operated with this (radical-enriched) gas mixture. Finally, we have observed that, for a straw tube operated at gain of a few times 10⁴, there is a contribution to the total collected charge coming from few, very large pulses, probably due to occasional transition to a limited streamer regime. (orig.)