The first NMR observation of the long-proposed Cu(III) intermediate in a substitution reaction of methylhalides with organocuprates is presented. One-dimensional H-1 and HMBC spectra give for the first time direct experimental evidence for a square planar coordination of the Cu(III) intermediate in solution. The presence of 4,4-dimethylcyclohexenone increases the detectable amount of the trimethylcyano Cu(III) species significantly, whereas in pure substitution reactions the proposed tetraalkyl Cu(III) species seems to be more stable in diethyl ether.