Two new cesium selenites containing TaO6 or TiO4F2 octahedra, namely, Cs(TaO2)(3)(SeO3)(2) (1) and Cs(TiOF)(3)(SeO3)(2) (2), have been prepared using standard high temperature solid-state method and hydrothermal reaction, respectively. Compound 1 crystallizes in P (3) over bar m1 and features an unusual [(TaO2)3(SeO3)(2)](-) sandwich-like double layer in which two [Ta(1)O-3(SeO3)](3-) layers are bridged by central Ta(2)O-6 octahedra via corner-sharing, whereas Cs(TiOF)(3)(SeO3)(2) with a polar space group P6(3)mc features an interesting hexagonal tungsten oxide (HTO) layered topology and presents a strong second harmonic generation (SHG) of about 5 x KDP (KH2PO4), which is much larger than those of A(VO2)(3)(QO(3))(2) (A = K, Tl, Rb, Cs, or NH4; Q = Se, Te) with a similar HTO layered structure. Cs(TiOF)(3)(SeO3)(2) is also type-I phase matching. The SHG of above-mentioned HTO materials can be enhanced greatly with the replacement of VO6 octahedra by TiO4F2 octahedra. Furthermore, thermal stabilities, UVvis diffuse reflectance spectra, infrared spectra, relationship between crystal structure and SHG, and theoretical calculations were also reported.