Abstract: A thermoformed sheet 1 for a plate-type gas-gas heat exchanger of the type comprising a case in which are stacked parallel plates spaced from each other by lateral spacing means, these plates having at their ends pointed regions provided with elements for alternately closing one of the two sides of the point, the case further comprising walls connected in a fluidtight manner to the pointed portions of the ends of the plates so as to form distinct inlet and outlet conduits for the gases which circulate in a countercurrent manner in the alternating spaces formed by the plates, this sheet being characterized in that it comprises a series of identical rows of elongated patterns 2a, 2b, 2c, 2d having edge portions folded on one side or the other of the sheet, each pattern comprising two parallel longitudinal edge portions 3a, 4a; 3b, 4b; 3c, 4c, 3d, 4d by which it is connected to the adjacent patterns so as to form a row and terminating in two pointed end portions 8a, 9a; 8b, 9b; 8c; 9c; 8d, 9d by which the patter