Direct comparison experiments are primarily used for measuring hydrogen/deuterium isotope effects in enzymatic reactions. The monoisotopic substrate and a deuterated form of the substrate are separately exposed to the enzyme of interest over a range of concentrations. The Michaelis-Menten kinetic parameters for both substrates are determined and the position-specific isotope effect at the site of deuteration is expressed as the ratio of the monoisotopic rate constant over the rare isotope rate constant.