This article reports on the stereo chemical aspects of the chemical stability of lactose solutions stored between 25 and 60 degrees C. The lactose used for the preparation of the aqueous solutions was a-lactose monohydrate with an anomer purity of 96% alpha and 4% beta based on the supplied certificate of analysis (using a GC analytical protocol), which was further confirmed here by nuclear magnetic resonance (NMR) analysis. Aliquots of lactose solutions were collected at different time points after the solutions were prepared and freeze-dried to remove water and halt epimerization for subsequent analysis by NMR. Epimerization was also monitored by polarimetry and infrared spectroscopy using a specially adapted Fourier transform infrared attenuated total reflectance (FTIR-ATR) method. Hydrolysis was analyzed by ion chromatography. The three different analytical approaches unambiguously showed that the epimerization of lactose in aqueous solution follows first order reversible kinetics between 25 to 60 degrees C. The overall rate constant was 4.4 X 10(-4) s(-1) +/- 0.9 (+/- standard deviation (SD)) at 25 degrees C. The forward rate constant was 1.6 times greater than the reverse rate constant, leading to an equilibrium constant of 1.6 +/- 0.1 (+/- SD) at 25 degrees C. The rate of epimerization for lactose increased with temperature and an Arrhenius plot yielded an activation energy of +52.3 kJ/mol supporting the hypothesis that the mechanism of lactose epimerization involves the formation of extremely short-lived intermediate structures. The main mechanism affecting lactose stability is epimerization, as no permanent hydrolysis or chemical degradation was observed. When preparing aqueous solutions of lactose, immediate storage in an ice bath at 0 degrees C will allow approximately 3 min (180 s) of analysis time before the anomeric ratio alters significantly (greater than 196) from the solid state composition of the starting material. In contrast a controlled anomeric composition (similar to 38% a and similar to 62% beta) will be achieved if an aqueous solution is left to equilibrate for over 4 h at 25 degrees C, while increasing the temperature up to 60 degrees C rapidly reduces the required equilibration time.</p