Phthalonitrile–polyhedral oligomeric silsesquioxane (POSS) copolymers were prepared by adding two different POSS cage mixtures: epoxycyclohexyl POSS (EP0408) and N-phenylaminopropyl POSS (AM0281). The cure behavior and properties of these polymers were analyzed and compared using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), Fourier transform far infrared (FTIR) measurements, and rheometric studies. The POSS-containing polymers showed higher chemical reactivity, better thermal stability and better mechanical performance in comparison to their unmodified counterparts. All the polymers showed water absorption below 1.5%. As revealed by FTIR measurements, the polymerization products contained triazine ring structures that were responsible for the superior thermal properties exhibited by these POSS-containing polymers.