Metal oxide clusters composed of group 5 metal ions, such as Nb and Ta, exhibit catalytic activities for CO2 fixation to styrene oxide (SO) due to the highly negative natural bonding charge of the terminal O atoms that could work as CO2 activation sites. In this study, tetrabutylammonium (TBA) salts of [TaxNb6−xO19]8− (TBA-TaxNb6−x, x = 0–6) were prepared and Ta-substitution effect on the catalytic properties of TBA-TaxNb6−x for CO2 fixation to SO was investigated. We found that TBA-Ta1Nb5 shows the highest styrene carbonate (SC) selectivity (95%) among TBA-TaxNb6−x, although the SO conversion monotonously increases with the incremental Ta substitution amount. The CO2 fixation to SO under various conditions and in situ X-ray absorption fine structure measurements reveal that CO2 is activated on both terminal O sites coordinated to the Ta (terminal OTa) and Nb (terminal ONb) sites, whereas the activation of SO proceeds on the terminal OTa and/or bridge O sites that are connected to Ta. Density functional theory (DFT) calculations reveal that the terminal OTa of TBA-Ta1Nb5 preferentially adsorbs CO2 compared with other ONb base sites. We conclude that the selective CO2 activation at terminal OTa of TBA-Ta1Nb5 without SO activation is a crucial factor for high SC selectivity in the CO2 fixation to SO.