Hybrid inorganic–organic materials are promising systems for a variety of applications due to their extraordinary properties from the combination of different building blocks. In this work, we present the synthesis and characterization of a hybrid material based on poly[acrylic acid] (PAA), 3-(trimethoxysilyl)propyl methacrylate (TMPM), and aluminum oxide (Al2O3). The synthesis was carried out using a two-step process: first, a polymerization via radical initiation, and subsequently, a sol–gel process. The hybrids were prepared by keeping constant the amount of acrylic acid and aluminum oxide precursor but changing the amount of TMPM. The physical and chemical properties of the hybrids were investigated using infrared spectroscopy (FT-IR), X-ray diffraction (XRD), N2 absorption (SBET), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The results indicate that all of the materials were simultaneous interpenetrating networks (SIPNs) and that the morphologies and the properties depend on the amount of TMPM used. All materials showed good thermal stability, and the surface area of the composite decreased as more TMPM was incorporated in the network.