Apparatus for cooling a bundle of capillary tubes to prevent overheating of the tubes during a parallel capillary electrophoresis procedure. The apparatus includes a jacket of thermally conductive solid material comprising mating jacket members movable between an open position in which the jacket members are separated to allow placement of the bundle in the jacket and a closed position in which the jacket members are mated together and the bundle is disposed inside the jacket in thermally conductive relation with the jacket. Window openings in the mating jacket members define a window to permit the passage of light through the window and the capillary tubes of the bundle at the location of the window.