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The composition of secondary organic aerosol (SOA) from the ozonolysis of C₅−C₈ cycloalkenes and α-pinene, as well as the effects of hydrocarbon precursor structure and particle-phase acidity on SOA formation, have been investigated by a series of controlled laboratory chamber experiments. A liquid chromatography−mass spectrometer and an ion trap mass spectrometer are used concurrently to identify and to quantify SOA components with molecular weights up to 1600 Da. Diacids, carbonyl-containing acids, diacid alkyl esters, and hydroxy diacids are the four major classes of low-molecular-weight (MW < 250 Da) components in the SOA; together they comprise 42−83% of the total SOA mass, assuming an aerosol density of 1.4 g/cm³. In addition, oligomers (MW > 250 Da) are found to be present in all SOA. Using surrogate standards, it is estimated that the mass fraction of oligomers in the total SOA is at least …