The analysis of temperature and pressure dependence of refractive index of liquid water is presented. The two-state outer-neighbour mixed bonding structural model was utilized for the analysis. The high accuracy of refractive index measurements were used to test the structural model. Simple linear dependence of refractive temperature and pressure of the components of the two-state model was in agreement with the experimental refractive index data. The pressure dependence allowed the calculation of the isothermal piezo-optic coefficient as a function of temperature.