THIN targets of oxygen-18, prepared by bombarding silver plates in an electromagnetic isotope separator at mass number 20 with water vapour in the ion source, were bombarded with protons (400–750 keV.) in a Cockcroft–Walton accelerator. The thin-target yield of α-particles within a cone of top angle 17° at 90° to the incident beam was studied with a proportional counter, the window thickness of which was such that only α-particles of range greater than 1.5 cm. in air could be detected. Gamma-rays had no influence upon the counter. The measured yield curve is given in the accompanying graph.