The charging of fine 'particles in unipolar coronas irradiated by soft X-rays (3.5-9.5 keV, lambda = 0.13-0.41 nm) was studied. Voltage-current characteristics were used to examine the corona inception voltage in presence and absence of X-ray irradiation and to estimate the ion concentrations. The capture characteristics of synthesized particles: iron oxide, sodium chloride, silica and titanium dioxide, were established in coronas, with and without X-ray irradiation. Enhanced charging of ultrafine particles in coronas was observed in conjunction with soft X-ray irradiation-A positive corona with X-ray irradiation resulted in the highest charging efficiencies, followed by a negative corona with X-ray, X-ray only, negative corona only and finally positive corona. only. Particle charging theories available in the literature were used to qualitatively explain the observed trends. (C) 2002 Elsevier Science Ltd. All rights reserved.