The birefringent properties of bisphenol Abased poly(phosphonate) and poly(thiophosphonate) were investigated. The orientational birefringences (C-R values) of the poly(phosphonate) derivatives are in the range from +1.2 X 10(-9) to +1.5 X 10(-9) Pa-1, which are less than half of those of the conventional bisphenol A-based polymers. In addition, their photoelastic birefringences (C-D values) are in the range from +5.7 X 10(-11) to +6.1 X 10(-11) Pa-1, which are also lower than those of the conventional bisphenol A-based polymers. From the DFT calculations of the polarizability anisotropies of the model compounds, the low birefringent properties of the poly(phosphonate) derivatives are most probably due to the presence of the P=O and P=S groups and the phenyl substituents perpendicularly oriented to the main chain direction.