Using a static analytical method, the solubilities of phosphonic acid, P,P'(1,4-piperazinediyl)bis-P,P,P',P'-tetraphenyl ester (PAPBE), N,N'-[1,3-phenylenebis-(methylene)]bis(phosphoramidic acid)P,P,P',P'-tetraphenyl ester (PNBE), and 4-(hydroxymethyl)-1-oxido-2,6,7-trioxa-1-phosphabicyclo [2.2.2] octane (PEPA) were measured from 293.15 K to 343.15 K in four selected organic solvents. Several thermodynamic models were used to correlate the experimental solubility data, such as Scatchard-Hildebrand, Wilson, nonrandom two-liquid (NRTL), and UNIQUAC. The calculated results showed that all the models reproduced the experimental data very well, and the UNIQUAC equation gives the best correlation. Using the group contribution methods, the solubility parameters of phosphorus flame retardants can be calculated, which were compared with the previous values calculated with the Scatchard-Hildebrand model.