In 2007/2008 the Institute for Reference Materials and Measurements (IRMM) organised an interlaboratory comparison (ILC) on the determination of 226Ra, 228Ra, 234U and 238U activity concentrations in mineral water. This paper describes the determination of the reference values for the activity concentrations of 234U and 238U by radiochemical separation and α-particle spectrometry performed at two independent laboratories. The experimental uncertainty of the reference values is discussed in detail.