Positron annihilation lifetime spectroscopy (PALS) has been performed on a series of PMMA nanohybrids containing nanometric TiO2, which were produced by means of different preparation methods, i.e., melt mixing, electrospinning combined with solution mixing, or in-situ sol gel growth methods, to study the effect of filler content and constituents on the free volume properties. The PMMA nanocomposites containing titania precursor or in-situformed TiO2 additives exhibit altered free volume properties compared to adding commercial TiO2 P25 fillers. The orthopositronium (o-Ps) lifetime (tau(3)) (free volume cavity size) was constant with composition in P25/PMMA nanohybrids due to the absence of interfacial interaction. However, in TiO2 precursor/PMMA composite fibers the free volume cavity size decreased substantially with hydroxyl group concentration and recovered after hydrothermal treatment. Additionally, a strong correlation between the glass transition temperature and the o-Ps lifetime in the nanohybrids was observed. These effects are caused by the hydrogen-bonding interaction between hydroxyl groups in the inorganic phase and carbonyl groups in the PMMA matrix, which concentration is dependent on the hydrothermal treatment, leading to differences in the packing of the polymer chains and a changed polymer segmental flexibility. The results also show a clear linear decrease in the o-Ps yield (I-3) with increasing P25 content of the composites. A dominant inhibition effect was observed in the TiO2 precursor/PMMA systems, caused by inhibition of positronium formation by the hydroxyl group in the titania precursor. In addition to the pronounced negative deviations of the o-Ps intensity with the concentration of hydroxyl groups in in-situ TiO2/PMMA nanohybrid fibers, a stronger inhibition efficiency of hydroxyl groups was observed than in the precursor/PMMA nanocomposite fibers.