A series of Cu SiO2 catalysts prepared by the ion-exchange method and containing 2.1 to 9.5 wt% copper are characterised in detail by TEM, X-ray diffraction analysis, XPS, and TPR TPO cycles. Two distinctly different copper species are identified, i.e., isolated copper atoms attached to two neighbouring silanol groups (ion-exchanged) representing 10-25% of the copper loading, and the remainder concentrated in numerous small copper particles (<6 nm, typically 1-3 nm diameter). In the calcined but unreduced samples, copper is present as both Cu(II) ions and copper oxide agglomerates of flat disc or hemispherical shape. Upon reduction in H2 and at temperatures up to 673 K, the ion-exchanged species reduce to Cu(I) only and retain their isolated nature. The CuO particles reduce to metallic copper. During reduction very little particle migration and agglomeration is observed, which we attribute to an interaction between the particles and the Cu(I)-modified support. Whereas the TPR results do not clearly show sequential stages in the reduction of CuO particles, the TPO experiments reveal three clearly defined reoxidation peaks which give information on the relative surface-to-volume ratios of the copper particles on the different catalysts. We suggest that the low deactivation rate for this type of catalyst is related to a strong interaction between the small copper particles and the ion-exchanged Cu(I) present on the SiO2 surface.