[en] This report presents results from online measurements with pH and Eh electrodes in water-saturated compacted MX-80 bentonite. The work included first testing and calibrating the electrodes in various standard solutions and in compacted bentonite where the conditions were adjusted by mixing into bentonite different chemical solutions before compaction. The developed methods were then used for pH and Eh studies in bentonite. The equipment for the measurements in bentonite included a measurement cell, measurement electrodes, a reference electrode, and a data-logging system. Eh was measured with Au and Pt wires as electrodes, while pH was determined with IrOx electrodes. The IrOx pH electrodes appeared to work well in different chemical solutions. The effects caused by the added chemical solutions in the bentonite were also clearly seen in the values of the measured pH. The pH values of the squeezed porewaters were quite similar to those measured directly in the bentonite. The recalibration of the electrodes after the measurements has shown that the electrodes are rather stabile even in the compacted bentonite. The pH measurements in the bentonite prepared from MX-80 and deionized water at different bentonite-to-water ratios suggest that in a closed system pH is buffered by calcite and the hydroxyl groups of montmorillonite. The modelling curve calculated with the geochemical code PHREEQC fits well the measured pH values. In an open system where the bentonite was in contact with an external solution, the effect of a high pH solution was seen in the pH measured at the depth of 5 mm in bentonite about 10 - 20 days later. It is probable that the hydroxyl ions diffuse into bentonite and react simultaneously with bentonite such that the increase of pH is delayed deeper in the sample. In strong redox standard solutions the Au and Pt electrodes give correct Eh values. In solutions without any clear redox pairs, the results given by Au and Pt electrodes tend to be different, with Pt usually giving lower values. When strong sulphide solution was mixed in bentonite, the measured values given by the Au and Pt electrodes were equal and close to the values measured in the squeezed porewaters. In an open system the change of the oxygen concentration in an external solution was quickly reflected as changes of redox potential inside the bentonite, to a depth of 5 mm. Oxygen seems to react quickly with bentonite such that the effect on Eh is weak deeper in the sample. At the moment, it is not clear which chemical reactions control the oxygen concentration in bentonite. (orig.)