Temperatures of a birefringent substrate (LiNbO3) have been measured by laser interferometry in a 13.56 MHz rf discharge. The measurement method is based on monitoring the variation of natural birefringence with temperature. The transient temperature rises and equilibrium values are investigated in nitrogen plasma at rf powers of ∼86 W. Owing to the large heat capacity per unit area of the substrate, the time constant of temperature rises is about ten times as large as the previously reported value of a polymer substrate (PMMA).