Blend membranes consisting of two polymer pairs improve gas separation, but compromise mechanical and thermal properties. To address this, incorporating titanium dioxide (TiO2) nanoparticles has been suggested, to enhance interactions between polymer phases. Therefore, the objective of this study was to investigate the impact of TiO2 as a filler on the thermal, surface mechanical, as well as gas separation properties of blend membranes. Blend polymeric membranes consisting of polyetherimide (PEI) and polyvinyl acetate (PVAc) with blend ratios of (99:1) and (98:2) were developed via a wet-phase inversion technique. In the latter, TiO2 was incorporated in ratios of 1 and 2 wt.% while maintaining a blend ratio of (98:2). TGA and DSC analyses were used to examine thermal properties, and nano-indentation tests were carried out to ascertain surface mechanical characteristics. On the other hand, a gas permeation set-up was used to determine gas separation performance. TGA tests showed that blend membranes containing TiO2 had better thermal characteristics. Indentation tests showed that TiO2-containing membranes exhibited greater surface hardness compared to other membranes. The results of gas permeation experiments showed that TiO2-containing membranes had better separation characteristics. PEI–PVAc blend membranes with 2 wt.% TiO2 as filler displayed superior separation performance for both gas pairs (CO2/CH4 and CO2/N2). The compatibility between the rubbery and glassy phases of blend membranes was improved as a result of the inclusion of TiO2, which further benefited their thermal, surface mechanical, and gas separation performances.