PNA dimers 1-5, containing either 4-thiothymine or N-3-methyl-4-thiothymine, were prepared, and the crystal structure of compound 3 was established. With regard to their photochemistry, none of these PNA analogues were able to fully mimic the photochemical behavior observed in the dinucleotide series. Whereas 1 and 2 displayed a sequence dependent photochemistry to give mainly 4-(alpha-thyminyl) adducts, their rearranged isomers 3 and 4 yielded mostly (5-4) photoadducts. Photoproducts originating from 3 and 4 were shown to undergo spontaneous reversal to their dithyminyl parent derivatives under acid conditions. Moreover, as a result of this photochemical study, it can be suggested that in solution, even at the dimer stage, PNAs adopt a conformation reminiscent of A-type DNA.