Disclosed are modified thymine bases that provide enhanced base-pairing affinity for adenine or 2,6-diaminopurine bases in polynucleotide hybridization complexes. Also disclosed are polynucleotide oligomers, polynucleotide hybridization complexes that comprise such modified thymine bases. Also disclosed are various methods of use. For example, in some embodiments, modified polynucleotide oligomers disclosed herein can be used as primers and probes for nucleic acid amplification and/or detection.