Reported are the synthesis and structural characterization of a new series of ternary rare-earth mix-chalcogenides RESTe (RE = Gd, Ho, Er, Tm) that have been obtained from high-temperature solid state reactions. These compounds crystallize in HoSTe structure types with monoclinic C2/ m and/or orthorhombic Immm space groups. The space group variation within this series is due to the position disorder along the Te plane (Te to TeA and TeB). The structural relationship and change between these two space groups are analyzed. It is realized that these compounds are all photocatalytic active under simulated sunlight. The trend of their photocatalytic activities and photocurrent responses is well-explained by using theoretical calculation as well as dipole moment analysis.