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‪This article examines how the quality of water consumed in Mons (Belgium) was measured and monitored in the nineteenth century as scientific knowledge expanded and new analysis methods became available. The water from public and private wells, poorly protected from contamination, is the only source of drinking water until 1870; its quality was rarely analyzed, nor was an essential factor behind the adoption of the spring catchment project in 1865. Until around 1890, the supply network progressed but the quality of the water consumed was not analyzed on the basis of measurable criteria. However, in the last decade of the nineteenth century, thanks to discoveries in bacteriology, analysis results prompted the council authorities to take the initiative of extending the network, including in the hitherto neglected working-class neighborhoods and outskirts of the city. These measures soon had a positive impact. Typhoid fever regressed significantly in Mons, testifying to a gradual improvement in drinking water quality.