UV-visible spectrometry is used here for on line analysis of heavy metals in industrial effluents. Heavy metals are analysed by use of a complexing agent giving coloured complexes with heavy metals. Unfortunately, the spectra of these complexes are early identical and only powerful multivariate analysis algorithm such as Partial Least Square (PLS) must be used in order to determine individual concentrations of 2 or 3 heavy metals in a solution. The calculation of the residuals of absorbance allows the detection of the events not present during the calibration, and so to eventually eliminate wrong measurements. On an industrial site, a model with the two components, Cu and Ni, was used and the main chemical problems which appeared are described and solutions are proposed.