Abstract: Baffle inserts or shims are disclosed for controlling the flow circuiting in plate or tube type heat exchangers of the kind having a plurality of stacked, hollow plate pairs or tubes including mating end bosses or areas or spacers having communicating openings formed therein to form a manifold for the flow of fluid through the plate pairs. The baffle inserts are located between preselected adjacent end bosses in the area of the end bosses only, to define different flow circuits through the plate pairs or tubes without having to use specially shaped plates or inserting obstructions into the flow passages inside the plate pairs or tubes. A method of using the baffle inserts is disclosed in which the baffle inserts are inserted during preassembly of the heat exchanger prior to the brazing operation.