Measurements of pH with fluorescein fluorescent intensity have been undertaken in order to measure the pH evolution of acidic solutions under pressure up to 250 MPa. This technique is quite rapid and allows to monitor changes of pH in real time. When the pressure increases, pH is shown to change even for buffers. Indeed, a pressure of 200 MPa increases the dissociation of orthophosphoric acid (Delta pH = -0.92) and acetic acid (Delta pH = -0.40). It also increases the dissociation of water (Delta pH = -0.31) and potassic buffers (Delta pH = -0.16 for potassium hydrophtalate and Delta pH = -0.28 for potassium dihydrogenophosphate). Nevertheless, the same pressure leads to a decrease in the dissociation of morpholineethanesulfonic acid buffer (Delta pH = + 0.50). Owing to these dissociation data against pressure, we have obtained the standard partial volume changed on ionization for the different acidic solutions used. The similarity in these values with the previous literature data is gratifying and supports the validity of the procedure used.