The response of a position-sensitive proportional counter filled with Ar + 30%CH 4 counting gas under a pressure of 7 atm has been studied in detail for 8-keV incident X-rays. The counter was operated in the region of limited proportionality, where the avalanche growth is greatly affected by self-induced space charge and hence an unfamiliar distorted energy spectrum is often observed. It is confirmed that the avalanches caused by the halo effect of Ar K X-rays, emitted in photoelectric absorption of incident X-rays, have higher gas amplification as compared to other photopeak avalanches due to different origin of primary electrons. The analysis has revealed that the effect of self-induced space charge hinders the avalanche growth of photopeak events to the extent of 50% of the observed avalanche charge. Comprehensive analysis of the complicated counter response in the region of limited proportionality under high pressure is provided.