The previously unreported complexes trans-[NBu4][RuX4(CNXyl)2] (X = Cl or Br, Xyl = 2,6-dimethylphenyl) have been prepared by treating [NBu4]2[RuX6] with the isocyanide ligand CNXyl in dichloromethane–ethanol and characterised by IR and UV-Vis spectroscopy, fast-atom-bombardment mass spectrometry, and elemental analysis (C, H, N and X). Their solution redox chemistry has been investigated using electrochemical and in situ spectroelectrochemical techniques. At low temperatures each complex undergoes a one-electron reduction to trans-[RuX4(CNXyl)2]2− (X = Cl or Br). At ambient temperature the same complexes undergo reduction in the presence of acetonitrile to afford mer,trans-[RuX3(CNXyl)2(NCMe)]−, which can be oxidised reversibly to mer,trans-[RuX3(CNXyl)2(NCMe)] (X = Cl or Br). Simulation of the cyclic voltammograms of [NBu4][RuX4(CNR)2] (X = Cl or Br, R = Xyl or But) in acetonitrile has enabled the rate constants for the formation of mer,trans-[RuX3(CNR)2(NCMe)]− to be evaluated. The rate constants were found to vary in the order X = Cl, R = Xyl < X = Br, R = Xyl < X = Cl, R = But < X = Br, R = But. The oxidation of trans-[RuX4(CNXyl)2]− (X = Cl or Br) in acetonitrile is accompanied by the reductive elimination of X˙. The number of product(s) formed is dependent upon the identity of the halide. For X = Cl oxidation ultimately leads to the formation of several species, which include mer,trans-[RuCl3(CNXyl)2(NCMe)] and trans,trans,trans-[RuCl2(CNXyl)2(NCMe)2]+, whereas for X = Br oxidation only produces mer,trans-[RuBr3(CNXyl)2(NCMe)]. All of the redox products have been characterised in situ by IR and UV-Vis spectroscopy in as many oxidation states as possible.