The ability to interconvert n-alkanes of different molecular weight has potentially great value in the context of natural sources or Fisher-Tropsch products. In the present study, we report the development of highly productive, well-defined, tandem catalytic systems for the metathesis of n-alkanes. Each system comprises one molecular catalyst (a "pincer"-ligated iridium complex) that effects alkane dehydrogenation and olefin hydrogenation, plus a second catalyst (molecular or solid-phase) for olefin metathesis. The systems all show complete selectivity for linear (n-alkane) product. We report one example that achieves selectivity with respect to the distribution of product molecular weights, in which n-decane is the predominant high-molecular-weight product of the metathesis of two moles of n-hexane. Additionally we have employed tandem dehydrogenation-olefin-metathesis catalyst systems to effect the metathesis-cyclooligomerization of cyclooctane and cyclodecane to give cycloalkanes with different carbon numbers, predominantly multiples of the substrate carbon number. Dimers are the major product, with successively decreasing amounts of higher oligomers formed. In addition, polymerization of the cycloalkanes occurs, presumably proceeding via olefin ring opening metathesis polymerization (ROMP).