N 6, N 9-Dimethyladenine ( N 6, N 9-Me 2Ade, 1) and N 1, N 4-dimethylcytosine ( N 1, N 4-Me 2Cyt, 3) were obtained by conventional methods, whereas the reaction of N 6-benzyladenine with MeI/NaOH resulted in the formation of N 3, N 6-MeBnAde ( 2a) and N 6, N 9-BnMeAde ( 2b). All compounds were fully characterized by microanalysis, NMR spectroscopy ( 1H, 13C) and 1, 2a·2MeOH and 3 also by single-crystal X-ray diffraction analyses. In single-crystals of 1, obtained from THF solutions, twofold N6–H···N7′ hydrogen-bonded dimeric units ( N 6, N 9-Me 2Ade) 2 (AA1 2 type according to Jeffrey and Saenger, 1991) were found. This proved to be another modification than that obtained by crystallization N 6, N 9-Me 2Ade from MeOH/PhCl (Sternglanz, 1978). Crystals of 2a·2MeOH exhibited an analogous hydrogen bond pattern as found in 1. The shorter N6···N7′ distance in 2a·2MeOH (2.932(2) Å) indicates slightly stronger hydrogen bonds than in 1 (3.078(3) Å). Crystals of 3 are built up from centrosymmetric dimers ( N 1, N 4-Me 2Cyt) 2 having a twofold N4–H···N3′ hydrogen bond, thus exhibiting the CC3 2 hydrogen bond pattern. The hydrogen bonding patterns in the dialkylated nucleobase derivatives are discussed in terms of those found in crystals of the less substituted nucleobases N 9-MeAde and Cyt/ N 1-MeCyt, respectively.