(1.3 x 10 -3 mol dm -3 h -1 ) instead of D2O (Figure ). This is in contrast to ferricyanide losses measured in the absence of 2,6-DHAQ, which were the same in D2O and H2O (Figure ). The lower ferricyanide loss rates observed in H2O relative to D2O could be explained as an inverse kinetic isotope effect, indicating that R-H bond breaking or formation is now involved in the ratedetermining step. As this effect was only observed after the addition of 2,6-DHAQ, it indicates that a new ferricyanide loss mechanism is involved when 2,6-DHAQ is present.