Gel structure of ethane-bridged trialkoxysilsesquioxane has been investigated extensively. In contrast, only a few studies have focused on the characterization of soluble ethane-bridged silsesquioxane polymer (sEBSP) because of difficult preparation and isolation processes. The present study aims to characterize the sEBSP and investigate its mixing effect in an organic polymer. The sEBSP was prepared by the hydrolysis–condensation of bis(trimethoxysilyl)ethane under a nitrogen flow and characterized by nuclear magnetic resonance spectroscopy (NMR), Fourier-transform infrared (FTIR) spectroscopy, and gel permeation chromatography (GPC). Based on the NMR and FTIR results, sEBSP was characterized as a randomly structured polymer with hybridized linear, cyclic, branched, and bicyclic units. It was mixed with poly(methyl methacrylate) (PMMA) and poly(bisphenol A-co-epichlorohydrin) (PBE) to form PMMA–sEBSP and PBE–sEBSP, respectively, which were then characterized by FTIR spectroscopy. The results confirmed the formation of hydrogen bonds between sEBSP and the organic polymer. The thermal stabilities of PMMA–sEBSP were better than those of the pure polymers. PBE–sEBSP exhibited a lower 5% weight loss temperature (Td5) because of the transalkoxylation between PBE and sEBSP. The miscibility of organic polymers and sEBSP was confirmed by differential scanning calorimetry (DSC). Based on their glass transition temperature, PMMA–sEBSP and PBE–sEBSP were classified as a hybrid and nanocomposite, respectively. Therefore, sEBSP was miscible to PMMA and PBE and affected to thermal properties.