The data set contains preliminary hourly mean values of the geomagnetic observatory St. Helena for 2010. The hourly means are calculated from minute means, which are based on 2 Hz raw data from an observatory grade, three component fluxgate magnetometer of type DTU FGE. The raw data is calibrated by means of weekly absolute measurements of the geomagnetic field at the observatory main pillar. The data set is provisional. Specifically, the data is visually controlled, but the applied baselines and formulas for calibration are not yet adopted in their final form. We expect the present preliminary data to be within a few nT of the final data that will eventually be distributed through the World Data Centres and INTERMAGNET.The data file is in ASCII format and contains blank separated first the month, day and hour of the time stamp of the data in the format MM DD HH followed by the hourly mean value of the geomagnetic field components X (North), Y (East) and Z (down) in nanotesla (nT). The time stamp is given for the beginning of the hour, for which the mean value of the field component is given.Geomagnetic observatories are described in e.g. Jankowski and Sucksdorf (1996). The St. Helena geomagnetic observatory was established by GFZ in 2007 and is operated by GFZ since. The network of geomagnetic observatories operated by GFZ and the observatories' instrumentation are described in Matzka (2016, http://doi.org/10.17815/jlsrf-2-136). Jankowski, J., and C. Sucksdorff, 1996, Guide for Magnetic Measurements and Observatory Practice, International Association of Geomagnetism and Aeronomy, Warsaw, 235 pp. ISBN 0-9650686-2-5, http://www.iaga-aiga.org/data/uploads/pdf/guides/iaga-guide-observatories.pdf