Concentrations of analytes can be determined within a few minutes using on-line analysis of the immunobinding kinetics in a solid phase immunoassay. This approach has been applied to the detection of atrazine. Atrazine is detected, at concentrations around the European Community limit (0.1 ¿g/l) by a competitive assay. To this end, the two channels of a Mach-Zehnder waveguide sensor are used simultaneously in a difference measurement. The advantage of this way of measuring is discussed with the atrazine measurements.