The synthesis of two novel PNA nucleobases derived from 1,8-naphthyridin-2(1H)-one (bT) and benzo[b]-1,8-naphthyridin-2(1H)-one (tT) are reported, together with their incorporation into oligomers of PNA and evaluation as substitutes for thymine. Compound bT is shown to be an effective mimic of the natural thymine nucleobase in PNA-DNA, PNA-RNA, and PNA-PNA duplex structures. A study using singly mismatched