Corrosion can occur in pipelines, especially in the consumer's plumbing. Corrosion products can result in dirty and rusty water, leading to customer complaints. It has also been suggested that corrosion favours bacterial proliferation due to rapid chlorine decay and coliform regrowth. Finally, corrosive water can lead to excessive lead and copper concentrations. Corrosion problems can result from mixing two different waters, often the case when using new water sources, and consequently corrosion control strategies are a concern for most utilities. One of the possibilities is to add soda or lime, although the latter has an undesirable impact on water hardness. The suburbs of Paris Water Utility is faced with the situation of having to blend nanofiltered, thus decarbonated water (37 mgd), with conventionally treated hard-surface water (53 mgd). A 1-year research project has been conducted with the following objectives: (1) selection of a model to calculate the suitable soda dosage; (2) determination of contact time; (3) testing the performances of the concept at pilot scale, especially during changes in flow regimes and water blending proportions; and (4) document possible interaction with the chlorine dosage. Experiments were conducted in the Mery-sur-Oise WTP from January through December 1996. The two different waters used were nanofiltered water with a calcium concentration of 60 mg/L-pH 6.5, and a "standard" water with a calcium concentration of 120 mg/L, pH 7.5. The water temperatures vary from 1 to 25 degreesC. The soda dosage is calculated using the Hallopeau-Dubin equations. These equations are used in a software package developed for automatic control systems. Experiments show that to obtain a stable pH, the necessary contact time can be as high as 7 min in cold-water conditions. In addition, it is also necessary to anticipate the hydraulic residence time. As a result, the software had to be modified to ensure that the saturation index remains positive in all situations. On-line records indicate that this goal was achieved on the pilot unit during 6 months of successful operation. Other experiments have shown that there might also be an interaction with the chlorine dosage, resulting in potential pH underestimation.