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The USP Apparatus II is the device commonly used to conduct dissolution testing in the pharmaceutical industry. Despite its widespread use, dissolution testing remains susceptible to significant error and test failures, and limited information is available on the hydrodynamics of this apparatus. In this work, laser-Doppler velocimetry (LDV) and computational fluid dynamics (CFD) were used, respectively, to experimentally map and computationally predict the velocity distribution inside a standard USP Apparatus II under the typical operating conditions mandated by the dissolution test procedure. The flow in the apparatus is strongly dominated by the tangential component of the velocity. Secondary flows consist of an upper and lower recirculation loop in the vertical plane, above and below the impeller, respectively. A low recirculation zone was observed in the lower part of the hemispherical vessel bottom where the tablet dissolution process takes place. The radial and axial velocities in the region just below the impeller were found to be very small. This is the most critical region of the apparatus since the dissolving tablet will likely be at this location during the dissolution test. The velocities in this region change significantly over short distances along the vessel bottom. This implies that small variations in the location of the tablet on the vessel bottom caused by the randomness of the tablet descent through the liquid are likely to result in significantly different velocities and velocity gradients near the tablet. This is likely to introduce variability in the test. © 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 96: 2327–2349, 2007 [ABSTRACT FROM AUTHOR]