A heat exchange unit for isothermal radial or axial-radial chemical reactors (1), with an overall configuration which is substantially annular and cylindrical having a passage (11) with a predetermined diameter running through it axially, is distinguished in that it comprises a plurality of substantially rectangular, flattened box-shaped heat exchangers (13), distributed in many coaxial and concentric arrays, defining in the unit (12) a plurality of radial alignments with a substantially radial arrangement, and wherein the exchangers (13) have long sides (13a) parallel to the axis of the unit (12) and short sides (13b) extending radially, each of the exchangers (13) comprising an inner chamber (17), intended to be passed through by an operating heat exchange fluid, at least one distributor pipe (19) and at least one collector pipe (20) of the operating fluid, associated with two respective opposing sides (13a) of the exchanger (13) and extending along them, the pipes (19, 20) being in fluid communication, on the one side, with the chamber (17) through at least one opening (19a, 20a) formed therein and, on the other side, with the outside of the exchanger (13), through respective inlet and outlet fittings (21, 22) for the operating fluid.