[formula: see text] The D-beta-ribo, D-beta-xylo, L-alpha-lyxo, and L-alpha-arabino members of the pentopyranosyl (4'-->2') oligonucleotide family show efficient intersystem cross-pairing among each other. This family of configurationally isomeric and conformationally well-defined pairing systems offers an opportunity to study structural factors that determine cross-communication between informational oligonucleotide systems of different backbone structure.