The etch rate of crystalline silicon in liquid solutions has been measured in situ by laser reflectance interferometry. A simple method has been developed which allows accurate real-time observation of the etch rate as a function of etching parameters and sample characteristics. The method is demonstrated by measurements on (100)single crystal silicon samples etched in tetramethylammonium hydroxide solutions. Etch rates in the range of 0.05-1 mu m/min have been measured. Some examples of the optical power reflected from silicon samples during etching are presented and the parameters which may influence the accuracy of the etch rate measurements are discussed. Interpretation of the interference pattern of the reflected power can be complicated due to sample thickness variations and surface roughness. To investigate the impact of surface topography on the detected optical power. three simulation models have been developed.