The samples under investigation are 20 mu m thick isotactic polypropylene films corona treated at different corona polarities and grid voltages in a three-electrode system. In order to control the results of the applied treatment a combination of two refractive index (RI) measuring techniques (laser refractometry and spectrophotometry) is used to study RI dispersion in the spectral range 400-1000 nm. It is found that the surface RI was lower than the volume RI. After a corona treatment both surface and volume RI increase. The changes of the volume RI as compared to surface values are more influenced by the corona treatment conditions: the greater the grid potential, the bigger the volume RI.