We report the chemical composition of PM10-associated water-soluble species in Mexico City during the second semester of 2000. PM10 samples were collected at four ambient air quality monitoring sites in Mexico City. We determined soluble ions (chloride, nitrate, sulfate, ammonium, sodium, potassium), ionizable transition metals (Zn, Fe, Ti, Pb, Mn, V, Ni, Cr, Cu) and soluble protein. The higher PM(10) levels were observed in Xalostoc (45-174 microg m(-3)) and the lowest in Pedregal (19-54 microg m(-3)). The highest SO2 average concentrations were observed in Tlalnepantla, NO2 in Merced and O3 and NO(x) in Pedregal. The concentration range of soluble sulfate was 6.7-7.9 and 19-25.5 microg m(-3) for ammonium, and 14.8-29.19 for soluble V and 3.2-7.7 ng m(-3) for Ni, suggesting a higher contribution of combustion sources. PM-associated soluble protein levels varied between 0.038 and 0.169 mg m(-3), representing a readily inhalable constituent that could contribute to adverse outcomes. The higher levels for most parameters studied were observed during the cold dry season, particularly in December. A richer content of soluble metals was observed when they were expressed by mass/mass units rather than by air volume units. Significant correlations between Ni-V, Ni-SO4(-2), V-SO4(-2), V-SO2, Ni-SO2 suggest the same type of emission source. The variable soluble metal and ion concentrations were strongly influenced by the seasonal meteoclimatic conditions and the differential contribution of emission sources. Our data support the idea that PM10 mass concentration by itself does not provide a clear understanding of a local PM air pollution problem.