The use of a Twyman-Green interferometer to measure the wedge angles and pyramidal error of high-precision optical prisms is described. The method allows these parameters to be determined independently or in combination. Prism angles are measured with the aid of a reference flat attached to a goniometer mounting. The pyramidal error is obtained by measuring the period of interferogram fringe pattern. Prism angles up to 25 deg have been measured with a standard deviation of 3 arcsec.