PM2.5 and PM10 samples were collected at four major cities in the Pearl River Delta (PRD), South China, during winter and summer in 2002. Six water-soluble ions, Na+, NH4+, K+, Cl-, NO3- and SO42- were measured using ion chromatography. On average, ionic species accounted for 53.3% and 40.5% for PM2.5 and PM10, respectively in winter and 39.4% and 35.2%, respectively in summer. Secondary ions such as sulfate, nitrate and ammonium accounted for the major part of the total ionic species. Sulfate was the most abundant specie followed by nitrate. Overall, a regional pollution tendency were shown in this campaign though there were higher concentrations of sulfate, nitrate and ammonium measured in Guangzhou City than those in the other PRD cities. Significant seasonal variations were also observed with higher levels of species in winter and lower in summer. The Asian monsoon system was favorable for removal and diffusion of air pollutants in PRD in summer while highly loading of local industrial emissions tended to deteriorate the air quality as well. NO3-/SO42- ratio indicates that mobile sources have considerably contribution to the urban aerosol but stationary sources should still not be neglected. Besides the primary emissions, complex atmospheric reactions under favorable weather conditions should be paid more attention for formulating control strategies for primary emission in the future in the PRD region.