The thymidine monomers of LNA with both alpha- and beta-configuration are incorporated with polarity reversals (i.e., with 3'-3' and 5'-5' junctions) in oligodeoxynucleotides with beta- and alpha-configuration, respectively. A 5'-O-phosphoramidite of the beta-LNA monomer is synthesised. Large destabilisations of duplexes with both complementary DNA and RNA are observed for oligodeoxynucleotides containing the alpha-LNA monomer, whereas a duplex with complementary RNA of an alpha-oligodeoxynucleotide containing the beta-LNA monomer is not destabilised.