Particle image velocimetry was used to characterize the flowfield on flat plates of aspect ratios 2 and 4 undergoing a starting rotation motion at Reynolds numbers, based on tip velocity, of 4,000, 8,000, and 16,000. The starting motion was performed in a tank of quiescent water. For both aspect ratios, a leading-edge vortex was observed on the suction surface of the plate, and its evolution and circulation were characterized with variations in angle of attack, Reynolds number, and azimuthal and spanwise positions. A region of strong counter-rotating vorticity was also observed between the leading-edge vortex and the plate surface, which becomes entrained by the leading-edge vortex. The circulation of the leading-edge vortex, when rendered dimensionless by the plate chord length and tip speed, was found to be relatively insensitive to Reynolds number and the azimuthal position of the plate within the ranges of the measurements; however, a strong (approximately linear) dependence on angle of attack was observed such that the leading-edge vortex circulation generally exceeded the bound circulation predicted by thin airfoil theory at the same local velocity. A significant collapse of the leading-edge vortex circulation was observed when normalized by the theoretical bound circulation, suggesting that this may be a useful tool for predicting leading-edge vortex strength.