A sensor apparatus and associated method for sensing and monitoring specific binding of analyte to an immobilized binding layer are disclosed. The apparatus preferably comprises an automatic critical angle refractometer having a linear scanned array and an optical system for illuminating a portion of the array, which illumination depends upon the refractive index of the binding layer deposited on an optically transparent element. The apparatus further includes a flow cell for bringing the analyte in contact with the binding layer. The apparatus also includes a computer for receiving and processing refractive index data from the critical angle refractometer during the reaction between the analyte and the layer, which computer may be peripherally connected to the refractometer or enclosed within the refractometer housing. A preferred sensing method of the present invention generally comprises providing a critical angle refractometer generating light impinging upon the immobilized binding layer, contacting the binding layer with a contacting phase, measuring the critical angle of total reflection, which measurements are indicative of the presence or absence of interactions between the analyte and the binding layer.