This invention provides compositions of matter that, when incorporated into an oligonucleotide, present to a complementary strand in a Watson-Crick pairing geometry a pattern of hydrogen bonds that is different from the pattern presented by adenine, guanine, cytosine, and thymine. Most specifically, this invention discloses and claims compositions of matter that present the same hydrogen bonding patterns as the isocytidine and isoguanosine nucleobases, but do not have unfavorable tautomeric forms, do not become disassociated from their sugar, and do not make major groove interactions, as much, as easily, or as strongly as isocytidine and isoguanosine.