The secondary ion energy spectrum of uranium depends on the experimental conditions, which is a problem for the application of a relative sensitivity factor (RSF) procedure for the measurement of the Nd concentration in UO2. This is interpreted as due to the dependence of the sputtering and ionisation yields of uranium on the oxidation state of the surface. Three experimental procedures are being tested to cope with this difficulty. (c) 2008 Elsevier B.V. All rights reserved.