All ATP coupled reactions, when performed at neutral or moderately alkaline pH, produce an acidification of the reaction mixture. The detection of small pH changes--0.1 mpH (1 mpH = 10(-3) pH)--in a constant buffering capacity solution makes it possible to quantify, over a wide concentration range (1-1500 mmol L(-1)), various analytes with very high precision and accuracy. Glucose, fructose, glycerol and gluconic acid can be analysed in less than 1 min with a single step reaction. Wine samples were analysed using the hexokinase reaction for glucose + fructose (sugars undergoing fermentation) and compared against an established method, showing excellent performance over the whole range of concentrations (R = 0.9994). Increased sensitivity in some applications can be obtained by cycling reactions, e.g. a kinase reaction followed by a phosphatase reaction. in a one step analysis, as required for lactulose assay in milk, a useful indicator of heat treatment damage. A sensitivity well below 0.1 mmol L(-1) in the original milk sample has been demonstrated.