A method for the measurement of fast, intensity-dependent refractive-index changes with the use of a modified Sagnac ring interferometer is presented. The measurement is not degraded by slowly responding background index changes. Nonlinear refractive-index changes in an undoped silicon wafer, and in poly-bis toluene sulfonate polydiacetylene and dye-doped polymethyl methacrylate waveguides, were measured with the use of a cw mode-locked Nd:YAG laser.