Two sensitive differential optical techniques useful for study of small reflectance or absorptive differences between two interfaces are described. The first scheme employs a mechanically chopped light beam impinging on an arrangement of symmetrical dual cells to allow detection of differential reflectances &Dgr;R/R<10−3. The second is a polarization‐modulated beam‐split laser version of the same method yielding approximately the same sensitivity. Representative measurements of voltage‐induced reflectance changes at liquid–solid interfaces using these methods are given.