A multicompartmental electrolyzer called membrane-separated wells for isoelectric focusing and trapping (MSWIFT) has been developed and tested for micropreparative-scale isoelectric trapping separations. In the MSWIFT, the length of the heat conduction path from the center of the compartments to the wall is less than 1 mm. The compartments are made from 99.8% nonporous alumina that has a high heat conductivity and a high specific heat capacity, leading to adequate joule heat dissipation even at power loads as high as 5 W The length of any compartment parallel to the electric field (the intermembrane distance) can be selected to be multiples of 1.5 mm, leading to compartment volumes that are multiples of about 60 PL. A maximum of 20 (1.5 mm. long) separation compartments can be readily assembled in the current version of MSWIFT. The MSWIFT has been used to desalt samples, isolate small ampholytic components (amino acids, peptides, and dyes), prefractionate complex protein mixtures and enrich minor components; these separations were achieved in 20-60 min.