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Square-planar cobalt diimine pyridine complexes LCoR (L=2,6-[RN＝CME]2C5H3N; R= n-C6H13 for Lhex, 2,6-(i-Pr)2C6H3 for Ldip) are active in the hydrogenation of monosubstituted and disubstituted olefins; sterically more hindered trisubstituted olefins do not react. For the LdipCoR system, a diamagnetic hydride intermediate was observed, although a small amount of paramagnetic product is also formed upon reaction of LdipCoR with H2. DFT studies suggest a traditional hydrogenation cycle starting with LCoH, except that intermediate LCo(R)(H2) transfers a hydrogen atom directly from H2 to the alkyl group in a σ-bond metathesis step, without going through a discrete CoIII intermediate. Autoclave experiments show that conversion is not linear in catalyst intake. Diffusion limitation was ruled out as an explanation, and we propose a concentration-dependent catalyst decay. At low catalyst intake conversion rates up to 2×104 (mol octene/mol Co/bar/h) can be reached. Reducing the steric bulk at the imine positions (Ldip →Lhex), or changing the metal from cobalt to rhodium, do not alter the activity or specificity of the hydrogenation much. For the LhexCo and LdipRh systems, no diamagnetic products corresponding to LdipCoH were observed. [Copyright &y& Elsevier]