An overview of the role of kinetic deuterium isotope effects (KD1E) in understanding hydrogen transfer reactions in organometallic chemistry is presented. Recent kinetic studies on the reaction of Os 3(CO) 10(μ-H) 2 with CF 3CN to form Os 3(CO) 10( μ− η 1-CF 3C(H)=N)- (μ-H) ( 1) and Os 3(CO) 10( μ- η 2-CF 3C=NH) (μ-H) ( 2) are reported The rate determining step in the reaction is formation of the initial adduct Os 3(CO)(CF 3CN)(μ-H)(H) which is present in insufficient concentration to observe directly. As a result, direct determination of rate constants for the hydrogen transfer step in the formation of 1 and 2 is not possible. However, variable temperature stems reveal that the ratio of 2:1 increases with decreasing temperature as does the KDIE for 2, but not for 1. These results, as well as the kinetic parameters calculated for the overall reaction, suggest that a significant tumteling component is associated with the formation of 2 but not 1.