A phase modulation system for a waveguide Mach–Zehnder interferometer sensor system has been developed which is capable of direct detection of optical phase changes smaller than 2.2&pgr; milliradians. When implemented in a chemical sensor interferometer system this corresponds to a sensitivity for refractive index changes of the analyte of better than 10−6, provided temperature variations are kept below 0.1 °C during the measuring period. The sensitivity of the instrument is demonstrated by a stepwise addition of low concentrations of glucose in water. The possibility to reset the instrument by a whole number of fringes makes it feasible to combine the high sensitivity with a virtually unlimited range. The accuracy of the detection system is independent of interferometer visibility changes during the measurement. This makes it possible to perform accurate measurements in solutions with varying degrees of opaqueness, such as milk, blood, or wastewater. These features are demonstrated with adsorption experiments of proteins to the underlying waveguide substrate. Also, the possibility to have the visibility as an extra parameter during the measurements can be of importance for further investigation of protein adsorption processes. ©1997 American Institute of Physics.