(1-Lu) to 570 mV (1-Gd), whereas a surface-confined controlled process would exhibit ΔEp = 0 V. Interestingly, the increase in ΔEp for process I at 250 mVs -1 roughly correlates with increasing RE 3+ ionic radius, with the smallest ΔEp of 110 mV obtained for 1-Lu, which contains the smallest Lu 3+ ion. Although these measurements are performed on immobilised solids of insoluble 1-RE, the nafion layer appears to play a key role in trapping the soluble oxidised product within the vicinity of the electrode. The electron transfer is diffusioncontrolled within the nafion layer with the smallest 1-RE compound diffusing most rapidly and affording the smallest ΔEp and greatest degree of electrochemical reversibility (Figure ).