Total sulfate, acidity, and principal cations were measured in aerosol samples collected from an aircraft over the northeastern United States and at ground level on filters protected by a denuder tube for NH3. Aerosol acidity was determined by aqueous extraction of the filters and titration with NaOH. Strong and weak acid components were resolved by using Gran plots, and for 30 fall and summer samples the weak acidity comprised x ± s = 26 ± 12% of the total acidity. The strong acidity averaged 25 ± 18% of the total sulfate equivalents. Hydrogen ion plus ammonium ion balanced closely the sulfate ion concentration with the ratio of equivalents of [(H+) + (NH4+)]/(S042_) = 1.03 ± 0.14 for 36 samples. A phase diagram for the H20-H2S04-(NH4)2S04 system was used to determine that the aerosol particles were liquid at the ambient relative humidity in 24 of 47 cases. For the 24 liquid samples the calculated pH averaged 0.11 ± 0.59.