AbstractA method is derived for the determination of acidity constants of ligands with two non‐identical acidic groups of similar acidity for the case of no interaction between these two groups. By this method the acidity constants of inosine‐5′‐monophosphate N(1)‐oxide are found to be pK = 5,43 and pK = 6,31. Spectrophotometric investigations show that pK belongs to the o‐hydroxy‐N‐oxide group. Therefore pK must be the acidity constant of the phosphate group. For comparison, the acidity constant of inosine N(1)‐oxide (pKA = 5,40) has also been determined.