Fermentations using pure and mixed cultures of Streptococcus salivarius subsp. thermophilus 404 and Lactobacillus delbrueckii subsp. bulgaricus 398 were performed at controlled pH, by means of a sodium hydroxide addition. Linear correlations between the substrate (lactose) and the products (galactose and lactic acid) concentrations and the equivalent sodium hydroxide concentration have been established. The relationships obtained from a statistical analysis were related to yield coefficients. As she sodium hydroxide weight measurements were continuously and reliably available, their use allowed the implementation of on-line indirect measurements of the lactose, galactose and lactic acid concentrations during the pure and mixed culture controlled pH fermentations. The lactose consumption rate and the lactic acid and galactose production rates were also obtained from on-line sodium hydroxide weight measurements. The agreement between the experimental and the calculated concentrations and rates of change was also studied.