One of the ways in which chlorine is thought to poison metal catalysts on oxide supports is by altering their dispersion. The effect of chlorine on Cu/ZnO(0001) model catalysts was studied by vapor-depositing Cu onto Zn-terminated ZnO(0001), both with and without preadsorbed Cl-2, using XPS, ion scattering spectroscopy (ISS), temperature-programmed desorption (TPD), work function, and band bending measurements. A disordered, but nearly close-packed overlayer of Cl adatoms forms at saturation with similar to 0.30 Cl adatoms per Zn site. Without Cl, vapor-deposited Cu grows in two-dimensional islands that cover similar to 33% of the ZnO, after which these islands thicken (i.e., as 3D Cu particles) while the clean ZnO between these Cu islands gets covered with Cu only very slowly. Preadsorbed Cl decreases the fraction of the surface that is covered by Cu islands by a factor of three, so Cl(a) either decreases the number of 2D Cu islands or their critical area before thickening. Both are consistent with weaker binding of Cu to the Cl-covered surface than to the clean ZnO. The TPD features for formate decomposition after HCOOH adsorption onto Cu/ZnO(0001) were suppressed with preadsorbed Cl, but the CO2 : CO selectivity increased. When Cu was deposited onto Cl-presaturated ZnO, neither the Zn- nor Cu-formate peaks were observed, showing that Cl covers both the Zn sites and the growing Cu islands, as suggested by ISS also.