We report results of high-flux experiments on tandem solar cells, with a real-sun probe predicated on mini-dish fiber-optic concentrators. Experimental results and their interpretation focus on: (a) a striking insensitivity of cell efficiency to flux map; (b) the predictability of the flux values at which cell efficiency peaks; and (c) performance of the same cell architecture at markedly smaller cell area. Copyright (C) 2006 John Wiley & Sons, Ltd.