Hybrid silica/poly(trimethylsilylpropyne) (PTMSP) membranes in which the silica particles are generated in the polymer solution with a sol gel reaction from a tetraethoxysilane (TEOS) precursor were fabricated. The membranes are characterized with microscopic, thermogravimetric, and pycnometric analysis; their sorption and diffusion properties with respect to n-C-4 and n-C-5 vapors are also measured at 25 degrees C. A significant reduction of the polymer specific volume, organic vapor solubility, and diffusivity is observed after incorporation of silica, according to a combination of free volume filling and compressive constraints. The behavior of the hybrid membranes (HM) is opposite that of mixed matrix membranes (MMM) obtained by physical mixing of preformed silica nanoparticles into PTMSP solutions, which show, on the contrary, free volume, vapor solubility, and diffusivity values higher than pure PTMSP. The hybrid membranes are characterized by larger silica domains than mixed matrix ones. Remarkably, however, the transport behavior of both types of membranes can be interpreted, by the same theoretical background, on the basis of a single structural parameter, namely the density of the polymer phase. This parameter allows a direct estimation of the variation of free volume and vapor solubility and diffusivity in both types of membranes, through a NELF/free volume approach.