K2RETa5O15 (RE=rare-earth element) fabricated using a flux method showed good activity on photoreduction of CO2 to CO with excellent selectivity toward CO evolution by using H2O as an electron donor. Display omitted •Flux method was applied to fabricate K2RETa5O15 for the first time.•Photoreduction of CO2 by H2O was achieved over Ag-modified K2RETa5O15.•The activity was found to be affected by the amount of KCl flux used. A facile one-pot flux method was applied to fabricate K2RETa5O15 (RE=rare-earth element) with tetragonal tungsten bronze (TTB) structures. Pure TTB structures of K2RETa5O15 (RE=La, Ce, Pr, Nd, and Sm) with rod-like morphologies were obtained using KCl as the potassium source and flux. For other potassium rare-earth tantalates, the TTB structure was dominant, with an additional RETaO4 phase present. Among all these K2RETa5O15, K2YTa5O15 gave the highest activity for photoreduction of CO2 with good selectivity toward CO evolution using H2O as an electron donor, even in the presence of additional YTaO4 phase. YTaO4 fabricated using the flux method under same condition was inactive for photoreduction of CO2 using H2O. It was found that the activity for the photoreduction of CO2 to be significantly affected by the weight ratio of potassium chloride to yttrium oxide and tantalum oxide (K/YT) in the fabrication process. The highest photoreduction activity achieved in CO evolution was 91.9μmolh−1 (for 1.0-g catalyst), with CO selectivity of 84.9%, at K/YT=1.0. Surface analysis of these tantalates fabricated with different K/YT ratios showed that a high ratio of surface yttrium to tantalum (Y/Ta) was important to achieve a high CO evolution rate.