Results of investigations into tandem ring-opening metathesis polymerization (ROMP)-hydrogenation are reviewed, in which hydrogen and 3-chloro-3-methyl-1-butyne provide simple chemical toggles to switch between metathesis and hydrogenation chemistry, enabling multiple tandem catalysis in chlorocarbon solvent. In the presence of methanol, hydrogenation of metathesis polymers can be carried out under 1 atm H-2. Issues of ligand design are examined in developing new Ru-diphosphine catalysts with improved selectivity, and an important decomposition pathway is identified for RuCl2 (PP)(CHR) systems (PP = chelating diphosphine). (C) 2002 Elsevier Science B.V. All rights reserved.