Substituted triazolopyrimidines of the formula I in which the substituents are as defined below: R1 is alkyl, halocycloalkyl, alkenyl, cycloalkyl, cycloalkenyl, halocycloalkenyl, alkynyl, haloalkynyl or naphthyl, or a five- or six-membered saturated, partially unsaturated or aromatic heterocycle comprising one to four heteroatoms from the group consisting of O, N and S, R2 is hydrogen, alkyl or one of the groups mentioned under R1; R1 and/or R2 may carry one to four identical or different groups Ra: Ra is chlorine, bromine, iodine, cyano, nitro, hydroxyl, alkyl, alkylcarbonyl, cycloalkyl, alkoxy, haloalkoxy, alkoxycarbonyl, alkylthio, alkylamino, dialkyl-amino, alkenyl, cycloalkenyl, alkenyloxy, haloalkenyloxy, alkynyl, halo-alkynyl, alkynyloxy, haloalkynyloxy, cycloalkoxy, cycloalkenyloxy, oxyalkylenoxy, naphthyl, a five- to ten-membered saturated, partially unsaturated or aromatic heterocycle comprising one to four heteroatoms from the group consisting of O, N and S, where these aliphatic, alicyclic or aromatic groups for their part may be substituted according to the description; and X is halogen; processes for preparing these compounds, compositions comprising them and their use for controlling phytopathogenic harmful fungi.