Comprehensive kinetic screening of Pd catalysts for Heck reactions via a consecutive pulse reaction methodology allows a more informed choice of catalyst for a particular transformation, taking into account not only initial reactivity but also longterm catalyst stability. Competitive reactions also offer mechanistic information. The resting state and rate-limiting step within the catalytic cycle are found to be the same for phospha-and azapalladacycles and ligandless Pd(OAc) 2 but different for the zero valent complex Pd[P(t-Bu) 3 ] 2 .With their seminal studies of Pd-catalyzed coupling reactions of aryl halides with olefins, Heck 1 and Mizoroki 2 set the basis for the development of one of the most useful catalytic methods for C-C bond formation, now known as the Heck reaction. In subsequent systematic investigations Richard Heck and his group clearly demonstrated the synthetic potential and broad scope of this method. 3 However, at that time many synthetic chemists were reluctant to use transition-metal-catalyzed reactions. In addition, considering the high price of palladium, it seemed unlikely that the Heck reaction would ever find industrial applications. However, the situation has dramatically changed since then. Impressive syntheses of complex molecules, using a Heck reaction as the key step, demonstrated the power of this method. At the same time, more efficient catalysts and improved catalytic procedures were developed. As a result, more and more chemists have become aware of the remarkable potential of the Heck reaction and the number of publications, reporting new applications and new catalysts, has increased enormously over the last decade. 4The search for highly active catalysts, which give high yields combined with high turnover numbers (TONs) and fast rates even with unreactive substrates, has been a major focus of recent research. A wide variety of efficient Pd catalysts with different ligands containing P, N, O, S, or C donor atoms has been reported. 4 Especially palladacycles derived from P-or N-functionalized aryl compounds have received much attention as stable, efficient precatalysts. 5 However, high TONs have also been obtained with simple inorganic Pd salts under optimized conditions. 4,6 A further significant step forward was made with catalyst systems based on electron-rich bulky alkylphosphines, which show high reactivity even toward aryl chlorides. 4,7 Ideally, for a meaningful evaluation of catalyst candidates, a comprehensive kinetic profile should be established, which provides detailed information about the catalyst performance during the entire course of the reaction and delineates the influence of individual reaction parameters. 8 Unfortunately, for most catalysts only limited information is available, such as catalyst loading, conversion and reaction time, and the reaction conditions used in different studies are seldom identical. Hence, based on literature data, it is often difficult to compare the efficiencies of different catalysts and to estimate a catalyst's suitability f...