A resonant cavity electro-optic phase modulator has been designed and implemented to operate at a data rate of 10 Mbps. The modulator consists of an electro-optic crystal located in a highly resonant cavity. The cavity is electro-optically switched on and off resonance, and the phase dispersion nearthe cavity resonance provides the output phase modulation. The performance of the modulator was measured by first heterodyne-detecting the signal to an intermediate frequency and then measuring the spectral characteristics using an rf spectrum analyzer. The measured phase shift is shown to be in good agreement with the theoretical predictions. Further theoretical analysis shows that the design of the modulator can be scaled to operate at 100 Mbps.