PM2.5 samples were collected at two locations in Guadalajara: Centro and Miravalle, during 2007. The first site (Centro) is located downtown and characterized by high vehicular traffic. Miravalle is in the southern part of the city, and influenced by emissions from high industrial and vehicular activity. Samples were collected for 24 h and the annual median concentrations of PM2.5 observed were 44.1 and 52.8 microg m(-3) at Centro and Miravalle, respectively. The concentration of PM2.5 observed at the Miravalle site was significantly higher (p < 0.002). Ca, Fe, Zn, Mg and Pb were the most abundant elements found at both sites. Miravalle showed higher annual concentrations of Ca, Pb, Cu, Cr, Sr, Ni, Mo, Fe, Mn, and Sb with levels of 1013.9, 74.9, 28.2, 9.4, 6.3, 4.4, 1.0, 628.0, 23.7 and 4.6 ng m(-3), respectively. At Centro and Miravalle quarterly and annual concentrations of Pb did not exceed 290 ng m(-3) and all values are well below those specified by air quality standards. The Principal Component Analysis (PCA), Cluster Analysis and the enrichment factor (EF) based on the concentrations of each element indicated that the main source of particulates at Centro was of geological origin, while Miravalle receives emissions from natural and anthropogenic sources. Both contribute to the chemical composition of PM2.5 in Guadalajara.