The solubility behaviors of poly(sulfonyldiphenylene phenylphosphonate) (PSPPP), a very efficient flame retardant for poly(ethylene terephthalate) (PET), in more than 50 solvents were examined. Its solubility parameters (δ) were determined by the intrinsic viscosity and turbidic titration methods. The two methods obtained consistent results, δ=21.0-21.6J1/2/cm and δ=21.0J1/2/cm3/2, and the three-dimensional solubility parameters were δ=18.9J1/2/cm3/2, δ=8.8J1/2/cm3/2, and δh=5.9J1/2/cm3/2. The miscibility of PSPPP with PET was estimated by the calculation of the heats of mixing, which were related to the difference between the solubility parameters of PSPPP and PET. Fourier transform infrared was used to examine the interactions between PSPPP and PET macromolecules, which were the internal factors of polymer-polymer miscibility. The results showed that PSPPP and PET were miscible within a very wide composition range, especially with less than 15wt% PSPPP, a composition of interest for the preparation of flame-retardant PET.