A microscale thermal field-flow fractionation (micro-TFFF) system has been designed, fabricated, and characterized. Motivation for miniaturization of TFFF systems was established by examining the geometrical scaling of the fundamental TFFF theory. Miniaturization of conventional macroscale TFFF systems was made possible through utilization of micromachining technologies. Fabrication of the micro-TFFF system was discussed in detail. The micro-TFFF system was characterized for plate height versus flow rate, single-component polystyrene retention, and multicomponent polystyrene separations. Retention, thermal diffusion coefficients, and maximum diameter-based selectivity values were extracted from separation data and found comparable with macroscale TFFF system results. Retention values ranged from 0.33 to 0.46. Thermal diffusion coefficients were between 3.0 x 10(-8) and 5.4 x 10(-8) cm2/s x K. The maximum diameter-based selectivity was 1.40.