Low-molecular-weight carboxylates (LMWCs) are an important group of water-soluble organic compounds found in atmospheric aerosols, which have significant implications for environmental and human health. However, the relative importance of the different sources of LMWCs is poorly documented and evaluations of their effects on the free acidity of aerosols are scarce. This study reports on five LMWCs (formate, acetate, oxalate, succinate, glutarate) found in fine particulate matter (PM_2.5) collected from two urban sites in southwestern China (Kunming and Nanning) from September 1–October 31, 2017. The formate-to-acetate ratio (FA/AA) was close to 1, and the main LMWCs were correlated with primary and secondary emission indicators. This suggests that the LMWCs had both primary sources and secondary sources. Quantitative calculations conducted using a positive matrix factorization model show that the secondary transformation of organic precursors contributed to more than 50% of the LMWCs. It suggests that these secondary formations are largely produced by gas-to-particle photochemical reactions base on the relationship between carboxylate and O₃. Calculations of total free acidity (TFA) showed that organic acids contribute a small proportion (<3%) and inorganic acids contribute most (>95%) to TFA, which may be related to huge emissions of gaseous pollutants (e.g., SO₂ and NO_x) and excess acidic ions in aerosols. Despite this, a linear relationship was found between inorganic or organic acid ions and the pH of PM_2.5. This indicates that both inorganic and organic acids affect aerosol acidity; inorganic acids reduce the pH of aerosols, while organic acids moderate low pH values.