This work presents the results obtained when determining the priority polycyclic aromatic hydrocarbon (PAH) concentration contained in particulate matter 10 μm in aerodynamic diameter or less (PM₁₀) and in the gas phase in Mexico City’s atmosphere by means of a field study conducted during 2005. PM₁₀ and vapor-phase PAHs were collected on prebaked quartz fiber filters and PUF-XAD-4 resin and quantified by gas chromatography (GC)-mass spectrometry. Vapor-phase PAHs comprised 86–97% of the total PAH mass, and naphthalene was the most abundant PAH determined, contributing 84–93% to the total mass. The benzo[a]pyrene content was on average 7% of the PAHs’ particle phase and 0.2–1% of the gas phase. The PAH concentrations measured where large industrial areas are located were 2–8 times greater during the fall/winter months (dry-cold season), when winds bear north- north-easterly, than in spring and summer. In contrast, the largest partition gas/particle ratio occurred during the dry-warm season (March–May). The results of statistical analysis using multivariate techniques suggest that motor vehicles are the main PAH contributors. The toxicity equivalence factors (TEFs) related to the carcinogenic benzo[a]pyrene potency were used for the assessment of the carcinogenic potential risk because of the measured airborne PAHs. Benzo[a]pyrene equivalent (BAPeq) concentrations were determined through multiplication of the individual PAH concentrations by their corresponding TEF value. The estimated total BAPeq for the whole field study considering only PM₁₀ PAHs was 1085 pg m^-3.However, when taking into account the PAHs in both phases, the estimated total BAPeq was 1250 pg m^-3; in either case, the values were greater than the 1000-pg m^-3 limit proposed by European countries.