Phosphate solubilising fungi (PSF) play an important role in increasing phosphorus (P) bioavailability and the fertility of soil. However, the mechanism by which PSF solubilise insoluble P using carboxylic acid is not fully understood. In this study,Penicillium oxalicumPSF-4 was isolated and shown to have satisfactory solubilisation performance towards tricalcium phosphate (TP) and iron phosphate (IP). In treatments with TP or IP, the soluble P (P-sol) concentrations were positively correlated with the carboxyl groups in organic acids but negatively correlated with the pK(sp)of insoluble P sources and pH. Additionally, the compositions of the organic acids (considering the lowest pK(a)value for acids with several pK(a)s) were remarkably different between the treatments: oxalic (pK(a) = 1.27), malic (pK(a) = 3.46), and formic (pK(a) = 3.75) acids in TP treatments (maximum 381 mg/L); and lactic (pK(a) = 3.85), gluconic (pK(a) = 3.86), and citric (pK(a) = 3.12) acids in IP treatments (maximum 1634 mg/L). The addition of H(+)withoutP. oxalicumPSF-4 inoculation markedly improvedP(sol)concentrations. The above results offer new insights that the pK(sp)of P sources, compositions of carboxylic acids, and pH regulation are the key factors influencing P solubilisation of different insoluble P sources.