A new 4-N-acylated deoxycytidine derivative, 4-N-(1H-pyrrol-2-ylcarbonyl)deoxycytidine, was synthesized and found to be stable under rather basic conditions. Oligodeoxynucleotides (ODNs) incorporating this modified deoxynucleoside at various positions were successfully synthesized by using a pivaloyloxymethyl (POM) group for protection of the pyrrole residue. The POM group was removed by treatment with 1.5 M NaOMe/MeOH. ODNs containing modified deoxycytidines exhibited hybridization properties superior to those of the unmodified ODNs. We found the acylation of the cytosine base with an aromatic acyl-type substituent led to significant increase of the thermo stability of DNA duplexes. This is the first noteworthy observation in this kind of modification. The synthesis and hybridization properties of 4-N-(1H-pyrrol-2-ylcarbonyl)deoxycytidine derivatives will be also reported.