The present invention relates to compounds of the formula I, the N-oxides, tautomers, the prodrugs and the pharmaceutically acceptable salts thereof: In formula I the variables X1 is CH or N, X2 is O or S and where R1, R2, R3, R4 and Q are as defined in the claims.The compounds of the formula I, the N-oxides, tautomers, the prodrugs and the pharmaceutically acceptable salts thereof are inhibitors of phosphodiesterase type 10A. Thus, the invention also relates to the use of the compounds of the formula I, the N-oxides, tautomers, the prodrugs and the pharmaceutically acceptable salts thereof for the manufacture of a medicament and which thus are suitable for treating or controlling of medical disorders selected from neurological disorders and psychiatric disorders, for ameliorating the symptoms associated with such disorders and for reducing the risk of such disorders.