An atomic force microscope was used to measure the forces acting between two polystyrene latex spheres in aqueous media. The results show an electrostatic repulsion at large separations which is overtaken by an attractive "hook" that pulls the two spheres into contact from a considerable range (20-400 nm), much larger than could be expected for a van der Waals attraction. The range of operation of this attraction varies from one experiment to another and is not correlated with electrolyte concentration. However, the range is found to decrease significantly when the level of dissolved gas in the water is reduced.