The intensity analysis along the crystal truncation rods has been used to analyse in situ the adsorption behaviour and the structure of K and Cs on Cu(0 0 1) at submonolayer coverages and room temperature. Up to about 0.25 ML K atoms adsorb in hollow sites followed by formation of a quasihexagonal superstructure. In contrast, for Cs adsorption the data can be interpreted by the formation of quasihexagonal Cs islands that grow with increasing coverage. For K an effective radius of 1.6(1) Å independent of coverage is determined. For Cs we fnd d = 2.1 (1) Å after formation of the quasihexagonal superstructure.