The first successful immobilization of a transition-metal catalyst inside MOFs by an acid−base reaction has been described. Simple absorption of a commercially available, amino-tagged, Hoveyda−Grubbs type ruthenium olefin metathesis catalyst inside an easy to make, Brønsted acidic MOF, (Cr)MIL-101-SO 3 H, yields a heterogeneous catalyst, which is stable even in very polar, "green" solvents, such as dimethyl carbonate and 2-propanol. The catalyst gives essentially ruthenium-free products upon simple filtration.