Oligonucleotides with a novel sugar-phosphate backbone containing at least one 2'-arabino-fluoronucleoside and an internucleoside 3'-NH--P(.dbd.O)(OR)--O-5' linkage, where R is a positively charged counter ion or hydrogen, and methods of synthesizing and using the inventive oligonucleotides are provided. The inventive phosphoramidate 2'-arabino-fluorooligonucleotides have a high RNA binding affinity to complementary nucleic acids and are base and acid stable.