With propan-2-ol as the donor, phosphorus ligand-substituted ruthenium clusters, [Ru₄H₄(CO)₈L₄], [L = PBuⁿ₃, P(OEt)₃, P(OMe)₃, PMe₃ or PPh₃], act as catalysts for the transfer hydrogenation of α, β-unsaturated aldehydes. High selectivities for the reduction of aldehydic functionalities rather than the olefinic ones are observed. Kinetic analyses indicate the existence of a dissociative and a non-dissociative pathway where the latter accounts for more than 99% of the reactions. On the basis of analytical solutions of the rate equations, a computer-assisted error-minimisation procedure yielded the optimum values of the different rate constants. With [²H₈]propan-2-ol as the donor, the cluster hydrides do not undergo any isotopic exchange and no isotope effect is observed for the non-dissociative pathway. From Arrhenius plots, activation energies for different ligand-substituted clusters are found to be in the order P(OMe)₃ > P(OEt)₃ > PBuⁿ₃.