Method and apparatus for mixing small volumes of reaction components. In one application, a plurality of removable vessels are placed in a parallel reactor, and reaction components are delivered to each vessel. An impeller is rotated in each vessel to mix the components. Each impeller has a rotary shaft and at least one blade on the shaft for placement in the reaction components. The blade is dimensioned so that one rotation of the impeller causes the blade to sweep out a sweep volume representing a large amount of the fill volume of the vessel.