Sulfonic nucleic acids were identified as inhibitors of ribonuclease A (RNase A). The incorporation of a strongly acidic group (sulfonic, –SO₃H) at the 3′-end of pyrimidine nucleosides thymidine and uridine was prompted by the low inhibition constant (K_i) values recorded for carboxymethylsulfonyl (–SO₂CH₂CO₂H) and –CO₂H functionalized nucleosides. It was envisaged that the sulfonic acid-modified pyrimidines would bind effectively with the positively charged P₁ site of ribonuclease A. Typical harsh conditions used for SO₃H incorporation were replaced with milder reaction conditions. The uridine analogue showing a K_i value of 0.96 μM elicited a better result than the thymidine-modified inhibitor. Notably, it was also the best result among all modified non-phosphate acidic nucleosides reported and screened so far as RNase A inhibitors.