Ambient PM2.5 samples were taken at 24 h intervals at two sites (Centro and Miravalle) in the city of Guadalajara from January to June 2008. The Centro site is located in the downtown, while the Miravalle site is located in an industrial zone south of the downtown. For both sites the higher concentrations of PM2.5 were between January and May. High correlation coefficients between sulfate, nitrate and ammonium of 0.95, 0.92 and 0.91, respectively, showed low variations in the concentrations of these species in the city. It was estimated that sulfate, nitrate and ammonium represented almost 47% of the PM2.5 mass in June at the Centro site, but in general the contributions in the other months were less than 21%, while at Miravalle this percentage was between 7.7% and 27.6%.