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Ground‐based studies of PM_2.5 were conducted for determination of 30 water‐soluble organic species, including dicarboxylic acids, ketocarboxylic acids and dicarbonyls, nine fatty acids, and benzoic acid, during the Campaign of Air Quality Research in Beijing 2006 (CAREBeijing‐2006; 21 August to 4 September 2006) at urban (Peking University, PKU) and suburban (Yufa) sites of Beijing. Molecular distributions of dicarboxylic acids demonstrated that oxalic acid (C₂) was the most abundant species, followed by phthalic acid (Ph) and succinic acid (C₄) at both sites. The sum of three dicarboxylic acids accounted for 71% and 74% of total quantified water‐soluble organics (327–1552 and 329–1124 ng m⁻³) in PKU and Yufa, respectively. Positive correlation was found between total quantified water‐soluble species and water‐soluble organic compounds (WSOC). On a carbon basis, total quantified dicarboxylic …