An experimental procedure is described for measuring the surface ionisation values phi(o) using thin-film measurements on a wide variety of substrates. The phi(o) values are determined by establishing the ratios of the film element X-ray intensities emitted from supported and unsupported thin films for a number of different film thicknesses and extrapolating towards a film thickness approaching zero. A number of 180 phi(o) data each for AlKalpha and PdLalpha X-radiations between 4 and 30 kV, on substrates ranging from beryllium up to bismuth, have been collected in this way. The purpose of this work was to provide a systematic database on which a variety of existing expressions for phi(o) could be tested. In the final assessment procedure, in which we also included 108 data from literature, we compared the performance of the various expressions from literature for phi(o), as well as a newly developed one of our own. Our final conclusion is that there is little to be chosen between the three highest-ranking expressions. (C) 2003 Elsevier Science B.V. All rights reserved.