Porosity is a very important property of iron ore green pellets. At LKAB, the mercury porosimeter was replaced some years ago by the GeoPyc instrument, in which the sample volume is measured by packing in silica sand. Some critical features of the GeoPyc measurement are pointed out in this study. The green pellets need to be strengthened by spraying with a fast-drying lacquer before measurement. A system for continuous instrument control is needed to detect any wear on the measuring tools that might lead to erroneous measurement results. The reproducibility measured on steel spheres was +/- 1% (2 sigma), which is equal to the reproducibility given by the instrument manufacturer. The reproducibility measured on lacquer-strengthened green pellets used as in-house reference samples was, however, much better, 0.3% (2 sigma). The calibration error against mercury porosimetry was improved from +/-0.6% to +/- 0.2% (2 sigma) after introducing the changes specified in the text. The GeoPyc instrument is easy to use and the problematic handling of mercury is avoided. (c) 2005 Elsevier B.V. All rights reserved.