The van der Waals and electrostatic interaction energies between a single particle and a flat plate were measured using the optical technique of total internal reflection microscopy (TIRM). The particles used were 15-mu m-diameter polystyrene latex spheres and the flat plate was a polished BK-7 glass slide. The measurements were performed in aqueous solutions of ionic strength ranging from 3 to 7 mM, and the particle-plate separation distances ranged from approximately 25 to 300 nm. During measurement, the particle was trapped in a secondary energy well formed by the repulsive electrostatic and attractive van der Waals forces; however, the particle was free to undergo Brownian motion at all times. These measurements, which capitalize on the extreme sensitivity of the TIRM technique, are believed to be the first direct measurement of the van der Waals interaction energy in aqueous solutions at separation distances where retardation effects are substantial. Comparison of the measured energy wells with predictions made with traditional energy equations produced only fair agreement; specifically, the measured well depths were consistently lower than predicted. However, when the measured results were compared with predictions made using the recent model of L. Suresh and J. Y. Walz ([J. Colloid Interface Sci. 183, 199 (1996)] for rough surfaces, very good agreement was obtained. The asperity heights yielding the best agreement ranged between 14 and 33 nm, with an average height of 26 nm. This value is consistent with previous estimates of the roughness height obtained by measuring the particle sedimentation velocity [J. Y. Walt and L. Suresh, J. Chem. Phys. 103, 10714 (1995)].