Tea distinct hydrogen adsorption sites on the copper component of a copper/alumina catalyst have been identified by monitoring the adsorption of hydrogen using in situ H-1 NMR spectroscopy. The NMR spectra reveal that one type of adsorption site is associated with relatively large, smooth copper particles whilst the other comprises a family of adsorption sites probably associated with faceting and defect sites on smaller copper particles. Exchange between the two types of adsorption site is surprisingly slow, with an upper Limit for exchange of 2500 s(-1), whilst exchange amongst the family of sites associated with the smaller particles, although still slow, is faster, of the order of milliseconds. Spillover exchange of hydrogen between the copper surface and the support is shown to be negligible on the timescale of the spin-lattice relaxation (tens of milliseconds). TPR, XRD, and optical and electron microscopy investigations of the catalyst precursors did not predict the differences between the catalyst samples.