Real time monitoring of the growth of plasma deposited transparent thin films by spectroscopic phase modulated ellipsometry (SPME) is presented. Two on‐line methods of determination of the refractive indexnand the film thicknessdare evaluated. The first one is based on the fast inversion of the Fresnel equations (0.2 s with a conventional PC 486 computer). Combining the measurements simultaneously recorded at two photon energies, a 5% relative precision is obtained on 5–6000 A˚ thick films even deposited at high deposition rate (32 A˚ s−1). On the other hand, a better precision can be obtained using a least squares fit procedure based on single photon energy SPME measurements. In the latter case, it is shown that the productndcan be determined with a 1% precision, up to 6000 A˚. ©1995 American Institute of Physics.