Previously described shipboard methods for measuring pH and Eh in cores have involved extrusion of the cores. A technique has been developed measuring pH and Eh of sediment cores inside a plastic liner. After the top of the core has been sealed, points of measurement are marked on the outside of the liner. At each point, a hole large enough to insert the electrode is drilled. By drilling the first hole at the bottom and working successively upward and by sealing each hole before, during, and after the measurements, the in-place characteristics of the interstitial water remains essentially unchanged.