It is shown thatinsituHeNe laser ellipsometric measurements performed during and after rf plasma exposure of a Si wafer with or without oxide can be used to obtain the wafer temperature during plasma exposure. The method utilizes either the temperature coefficient &dgr;n/&dgr;Tof the refractive index of Si or the linear thermal expansion coefficient &dgr;l/l&dgr;Tof SiO2. The values of these parameters have been redetermined in this work.