Al-4 wt.%Cu alloy has been gas atomized using a commercial close-coupled gas atomization system. The resulting metal powders have been sieved into six size fractions and the secondary dendrite arm spacing has been determined using electron microscopy. Cooling rates for the powders have been estimated using a range of published conversion factors for Al-Cu alloy, with reasonable agreement being found between sources. We find that cooling rates are very low compared to those often quoted for gas atomized powders, of the order of 10 4 K s -1 for sub-38 μm powders. We believe that a number of numerical studies of gas atomization have overestimated the cooling rate during solidification, probably as a consequence of overestimating the differential velocity between the gas and the particles. From the cooling rates measure here we estimate that such velocities are unlikely to exceed 20 m s -1 .