Highly oxygenated compounds assigned to be oxidation products of α‐pinene have recently been observed in substantial concentrations in ambient aerosols. Here, we confirm the unknown α‐pinene tracer compound with molecular weight (MW) 204 as the C8‐tricarboxylic acid 3‐methyl‐1,2,3‐butanetricarboxylic acid. Its gas and liquid chromatographic behaviors and its mass spectral characteristics in electron ionization and negative ion electrospray ionization perfectly agree with those of a synthesized reference compound. The formation of this compound is explained by further reaction of cis‐pinonic acid involving participation of the OH radical. This study illustrates that complex, multi‐generation chemistry holds for the photooxidation of α‐pinene in the presence of NOx.