Adenyldiyl trimers with different kinds of substituents on 2 -5 and 3 -5 phosphate linkages have been synthesized in a general, regiodefined manner. Examination of the 2D NMR spectra reveals that the trimers with adenylyl(2 -5 )adenosine linkage make a syn-anti as well as syn-syn base stack between the two adenyl bases and exist as a mixture of the two conformers.