We investigated the structural and optical properties of (A(0.6)X)Bi-4(Nb0.6W0.4)O8X (A = K, Rb, Cs and X = Cl, Br). We found that these Sillen-Aurivillius-related oxyhalides have band gaps of ca. 3.1 eV (X = Cl) and ca. 2.8 eV (X = Br), which are surprisingly larger than 2.4 eV for the archetypal Bi4NbO8Cl, meaning a substantial stabilization of O-2p orbitals upon A(0.6)X layer insertion. The present study suggests versatile possibilities of band gap engineering in Sillen-Aurivillius type compounds.