© Taiwan Association for Aerosol Research.Intensive field measurements of formaldehyde (HCHO) and acetaldehyde (CH3CHO) were concurrently conducted at a mountain site (TMS) and an urban site (TW) at the foot of the same mountain in Hong Kong from September to November 2010. The spatiotemporal variations of HCHO and CH3CHO, the correlation between HCHO and CH3CHO and the ratios of HCHO/CH3CHO indicated different impacts of primary emissions and secondary formation at the two sites. The source apportionments of HCHO and CH3CHO at both sites were investigated using the Positive Matrix Factorization (PMF) model, while the in-situ formation of HCHO and CH3CHO was estimated using a Photochemical Box Model coupled with Master Chemical Mechanism (PBM-MCM). At TMS, the in-situ formation was the most significant contributor to ambient HCHO and CH3CHO, accounting for 51 ± 5 and 32 ± 2%, respectively, followed by the transport of photochemicallyformed aldehydes, vehicular emissions, biogenic emissions, biomass burning and solvent usage. On the other hand, at TW, the in-situ formation and vehicular emissions explained 55 ± 7% and 18 ± 1% of ambient HCHO, respectively, while vehicular emissions and in-situ formation made comparable contributions to CH3CHO (~35%). The findings are helpful for the formulation and implementation of appropriate control strategies for aldehydes and their precursors in Hong Kong.