ECM data from the GRIP core obtained with the technique described by Hammer (1980). People involved in the measurements include Henrik B. Clausen, Dorthe Dahl-Jensen, Christine Hvidberg, Niels Gundestrup, Steffen B. Hansen, P. M. Kristinsdottir, Jakob Schwander, and J.P. Steffensen. Data were recorded in parallel on paper in high resolution and digitally in 1 cm resolution. The main core was drilled during the GRIP project in 1989-1992 at the Summit of the Greenland ice sheet. The length is 3025 m. Present-day accumulation 0.23 m ice/yr. Data from the top 101.3 m comes from the S3 shallow core, but the two data sets have been fully integrated on a common depth scale. The data are given as H+ ion concentration versus depth (measured from the undisturbed surface in 1989), but the absolute calibration (H+ = 0.045 I^1.73) must be considered tentative. For more information on the measurements and calibration, see Hammer 1980 and Clausen et al. (1995). Please cite the Hammer (1980), Clausen et al. (1995), and Rasmussen et al. (2023) when using this data file.