This paper reports the data obtained concerning the determination of pH of acetic acid, citric acid, oxalic acid and tartaric acid solutions from 0,005 M to 0,5 M. There is a linear relationship, between pH and pC log 1/C, where C is the concentration in molarity, of studied acids. On the other hand, the calculation of pH through a quadratic equation, using only the first thermodynamic constant, furnishes data very close to the determinated values. The calculated pH of acetic acid solutions (from 0,55 to 0,0055 M) of citric acid solutions (from 0,25 to 0,025 M) and of tartaric add solutions (from 0,25 to 0,05 M) through an equation of first degree, using only the first thermodynamic constant, gave results which are also very close to the determined pH value. It was possible to establish two equations for the line of regression of pH on pC for each acid. In the first equation the determined pH is the dependent variable and in the second equation the calculated pH is dependent variable. The independent variable in both equations is pC.