The objectives of this work were to characterize and compare the chemical composition of the water-soluble fraction of the PM₁₀ particles (Dp < 10μm) in two sites: one inside the Metropolitan Area of São Paulo (MASP) and another, 250km apart, inside the State Park of Serra do Mar (CUNHA) part of the Atlantic Forest Reserve, both located in São Paulo State, Brazil. The atmospheric particles were collected during dry and wet season. The morphologic parameters of the particles were characterized for the different size fractions of the collected material. In the aqueous extract of the particulate fine fraction the major ions (Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl-, NO₃ -, NH₄ ⁺, SO₄ ²-) and trace elements (Al, Mn, Fe, Pb, Cd, Zn, Ti, Ni, Cu, Co, Ba) were determined. The morphological characteristics of the particles collected within the MASP are typical of polluted environment while in CUNHA there is no evidence of this type of contribution. Regarding the solute concentrations it was observed that the most abundant major ions and trace elements were K⁺, Ca²⁺, Na⁺, Cl- and Pb, for CUNHA and NO₃ -, SO₄ ²-, NH₄ ⁺ and Mn, Ni, Pb, Co, Cd and Ba for MASP. These differences are associated with the different sources of the particles. In the urban area they are predominantly of pollution origin, mainly from vehicle emissions, and road dust suspension, while in the State Park they are mainly of biogenic, terrigenous and oceanic origins. For these reasons the CUNHA region can be considered to be a regional reference site for studies concerning eventual disturbances in the Cunha background site, derived from transported pollution.