In the photochemical machining of micro-parts from nickel-containing alloys, a solution of ferric chloride is used as the etchant. As the alloy is dissolved into solution, the concentration of dissolved nickel will increase to a point where the surface finish of the etched parts will be affected adversely and become rougher. To control part quality and reduce the number of defective parts, it is therefore necessary to monitor the build-up of nickel ions on-line. A miniature, commercial microspectrometer has been evaluated for its suitability to monitor the nickel concentration. The microspectrometer has been shown to be costeffective and is capable of measuring the concentration of nickel ions to ± I g/1.