Measurements of the electro‐optic properties of high‐resistivity GaAs epitaxial thin films have been made at the 10.6‐&mgr;m CO2laser wavelength by a novel rf interferometric technique. The described technique affords very accurate and convenient measurements of the phase shift of a 10.6‐&mgr;m guided wave mode in thin films as a function of reverse‐biased junction voltage. The measured electro‐optic coefficient for thin films is found to be in good agreement with conventional measurements,r41= 1.2 × 10−10cm/V at 10 &mgr;m, for bulk GaAs electro‐optic crystals.