2-Mercapto-substituted triazolopyrimidines of the formula I in which the substituents are as defined below: L is halogen, cyano, nitro, alkyl, alkenyl, alkynyl, haloalkyl, haloalkenyl, alkoxy, alkenyloxy, alkynyloxy, haloalkoxy or —C(═O)-A; A is hydrogen, hydroxyl, alkyl, alkenyl, alkoxy, haloalkoxy, alkylamino or dialkylamino; m is 0, 1, 2, 3, 4 or 5; X is halogen, cyano, alkyl, haloalkyl, alkoxy or haloalkoxy; R1, R2 are hydrogen, alkyl, haloalkyl, cycloalkyl, halocycloalkyl, alkenyl, alkadienyl, haloalkenyl, cycloalkenyl, alkynyl, haloalkynyl or cycloalkynyl, phenyl, naphthyl or a five- to ten-membered saturated, partially unsaturated or aromatic heterocycle which contains one to four hetero atoms from the group consisting of O, N and S; R1 and R2 together with the nitrogen atom to which they are attached may also form a five- or six-membered ring which may be interrupted by an atom from the group consisting of O, N and S; where R1 and/or R2 may be substituted as stated in the description; processes for preparing these compounds, compositions comprising them and their use for controlling phytopathogenic harmful fungi are described.