Water-soluble organic compounds in aerosol contribute a significant fraction to organic aerosol mass and influence interactions of aerosols with water in the atmosphere. Despite their importance, the sources and formation mechanisms of these compounds are not well understood. In this work, we measured the size distributions of water-soluble organic carbon (WSOC) and its most abundant single component, oxalate, in the urban area of Shenzhen, a coastal metropolitan city in southern China. In the cloud condensation nuclei size range, organic compounds were found to constitute a significant fraction (roughly one half) of the total water-soluble substance mass. The positive matrix factorization (PMF) model was used to resolve the bulk mass size distributions into condensation, droplet, and coarse modes, with their respective modal peak at 0.4, 1.0, and 5.5 ¿m. Both WSOC and oxalate had a dominant droplet mode, a minor condensation mode, and a minor coarse mode. Approximately one half of WSOC and two thirds of oxalate mass were in the droplet mode. The sources and formation mechanisms of oxalate and WSOC were inferred in reference to the well-understood size distribution characteristics of inorganic species (Ca2+, Na+, K+, and SO42-), in conjunction with source identification and contribution estimation by PMF. We found that the droplet mode oxalate was mostly produced from in-cloud aqueous phase reactions. Among significant sources contributing to the total WSOC were biomass burning, in-cloud processing, soil dust particles, and aged sea salt particles. The first two sources were the major contributors to the droplet WSOC, while the latter two were responsible for the coarse mode WSOC. The droplet mode WSOC correlated well with K+ and sulfate, consistent with the source estimates by PMF. Future work on WSOC is suggested to be directed at characterizing the biomass burning aerosols and elucidating the molecular formation pathways in the aqueous phase.