The autoprotolysis constant K-HS of formic acid/water mixtures as solvent has been calculated from acid-base potentiometric titration curves. A correlation of the acidity scale pK(HS) of each medium versus pure water has been implemented owing to the Strehlow R-0(H+) electrochemical redox function. The results show that formic acid/water mixtures are much more dissociated than pure water; such media are sufficiently dissociated to allow electrochemical measures without addition of an electrolyte. It has also been shown that for a same H+ concentration the activity of protons increases with formic acid concentration. For more than 80 wt.% of formic acid the acidity is sufficiently increased to locate the whole acidity scale pK(HS) in the super acid medium of the generalized acidity scale pH(H2O). (C) 2007 Elsevier Ltd. All rights reserved.