A simple model of the effect of optical activity on two-wave mixing coupling constants in Bi₁₂SiO₂₀ (BSO) crystal is developed. The model is tested for two particular transverse electro optic crystal geometries with supporting experimental results. It is revealed that for one configuration non zero coupling will be obtained which, as interaction length increases, becomes independent of the angle of linear polarisation of interacting beams. A second configuration gives a coupling constant which alternates between equal positive and negative values as input beam, polarisation angle is varied. The degree of coupling in this latter case reaches a maximum determined by optical activity parameter, ρ, but is ultimately independent of interaction length.