The J-mixing effect in the (2S+1)LJ level barycentre energy and splitting of trivalent lanthanide ions is analysed. The main predictions are that the maximum (2S+1)LJ level splitting (DE) is reduced and the barycentre energy can become lower with increasing DE. This effect is particularly important in low symmetries. Satisfactory agreement between the predictions and experimental data was obtained to all analysed systems, namely, the maximum F-7(1) level splitting of the Eu3+ ion and its barycentre energy in crystals and glasses. (C) 2011 Elsevier B.V. All rights reserved.