Soluble metal-ligand complexes are useful as catalysts for many organic reactions. The large number of metals and ligands available suggests a combinatorial approach to catalyst discovery. Carrying out reactions in very small (microliter) volumes in capillaries has many advantages in this regard, including material conservation, isolation from the atmosphere, and ease of transport of species by using pressure-induced flow. We have developed a capillary reactor in which separate zones of catalyst and reactants are combined and react. Zones are loaded serially into the capillary reactor from an autosampler, they react in parallel in the capillary reactor (at elevated temperature) and are ejected serially and under computer control for analysis by online GC. Offline analysis following sample collection is also possible. The Stille cross-coupling reaction has been the focus of our recent activity. Known palladium-based precatalysts and phosphine or arsine ligands were screened to validate the approach taken here. The results largely agree with results obtained by traditional organic synthesis, validating the method. The throughput of the nonoptimized system is over two 5-h reactions/h. For example, 40 5-h reactions examining the effect of catalyst loading were performed in 2 9-h runs requiring a total of 2 h of operator time.