Nanocomposites membranes represent a promising alternative to the limitations of organic and inorganic materials for high productivity and selectivity gas separation membranes. The previously developed concept of emulsion-polymerized mixed matrix (EPMM) membranes was further advanced in this book by investigating the effects of surfactant and compatibilizer on inorganic loading in poly(2,6-dimethyl-1,4-phenylene oxide) (PPO)-based EPMM membranes, in which inorganic part of the membranes originated from tetraethylorthosilicate (TEOS). The polymerization of TEOS, which consists of hydrolysis and condensation of TEOS, was carried out as one- and two-step processes. The synthesized EPMM membranes were characterized using 29Si nuclear magnetic resonance (29Si NMR), differential scanning calorimetry (DSC), inductively coupled plasma mass spectrometry (ICP-MS), and gas permeation measurements carried out in a constant pressure (CP) system. This book has provided the foundation of further research in emulsion-polymerized mixed matrix (EPMM) membranes.