Abstract Recent experimental studies have identified cis-pinic acid (a C 9 dicarboxylic acid) as a condensed-phase product of the ozonolysis of both α- and β-pinene, and it is currently believed to be the most likely degradation product leading to the prompt formation of new aerosols by nucleation. The observed timescale of aerosol formation appears to require that cis-pinic acid is a first-generation product, and a possible mechanism for its formation has therefore been developed. The key step in the proposed mechanism requires that the isomerisation of a complex C 9 acyl-oxy radical by a 1,7 H atom shift is able to compete with the alternative decomposition to CO 2 and a C 8 organic radical: Thermodynamic and kinetic arguments are presented, on the basis of semi-empirical electronic structure calculations, which support this proposed mechanism, and thereby the competition between the two pathways. The transfer of the labile aldehydic H atom is shown to be especially facile in this case because it occurs though an unstrained transition state; this feature can in turn be attributed to the cis-substitution of the four-membered ring, which enforces the steric proximity of the acyl-oxy and aldehyde groups. The mechanism can explain the formation of cis-pinic acid from both α- and β-pinene, because the acyl-oxy radical is likely to be formed following the decomposition of excited Criegee biradicals formed in both systems. It is also possible that a similar isomerisation reaction of a complex C 10 α-carbonyl oxy radical by a 1,8 H atom shift might explain the very recently observed formation of cis-10-hydroxy-pinonic acid from α-pinene ozonolysis, and this possibility is also explored. An existing detailed scheme describing the degradation of α-pinene (part of the Master Chemical Mechanism, MCM) is updated to include the proposed cis-pinic acid and cis-10-hydroxy-pinonic acid formation mechanisms, and the values of several uncertain parameters are adjusted on the basis of reported yields of a series of organic products from the ozonolysis of α-pinene. The updated degradation scheme is incorporated into a boundary layer box model, and representative ambient concentrations of the organic acids and other oxygenated products are calculated for a range of representative conditions appropriate to the boundary layer over central Europe. The simulated concentrations of the organic acids in general, and cis-pinic acid in particular, are strongly dependent on the level of NO X , and suggest that new aerosol formation from the oxidation of α-pinene is likely to be more favoured at lower NO X levels.