Although bismuth chalcohalides, such as BiSI and BiSeI, have been recently attracting considerable attention as photovoltaic materials, the methods available to synthesize them are quite limited thus far. In this study, a novel, facile method to synthesize these chalcohalides, including BiSBr1-xIx solid solutions, at low temperatures was developed via the substitution of anions from O(2-) to S(2-) (or Se(2-)) using bismuth oxyhalide precursors. Complete phase transition was readily observed upon treatment of BiOI particles with H2S or H2Se at surprisingly low temperatures of less than 150 °C and short reaction times of less than 1 h, producing BiSI and BiSeI particles, respectively. This method was also applied for synthesizing BiSBr1-xIx, where continuous changes in their band gaps were observed depending on the ratio between iodine and bromine. The composition of all elements (except oxygen) in the chalcohalides thus produced was almost identical to that of the oxyhalide precursors, attributed to the suppressed volatilization of halogens at such low temperatures. All chalcohalides loaded on FTO clearly exhibited an anodic photocurrent in an acetonitrile solution containing I(-), attributed to their n-type nature, e.g., the BiSI electrode exhibited high IPCE (64% at 700 nm, +0.2 V vs. Ag/AgCl).