A system for controlling the pH of a formaldehyde-free aqueous binder composition used in the manufacture of glass fiber products. Following the formation of an aqueous binder composition, acid is added to the composition to reduce the pH to less than about 3.4. After the acid is combined with the aqueous binder composition, the pH of the composition is measured and then compared within a pre-set set point. The rate of acid addition is then adjusted thereby adjusting the pH of the composition such that the measured pH equals the set point pH. The pH measurement is preferably done before the addition of other additives to avoid interference by those additives.