CuI single crystals were grown in silica gel with decomplexation method modified by concentration programming. The influence of the concentration and its gradient of CuI (.) HI complex in silica gel on the crystal growth was studied by measuring concentration of copper ions with spectrophotometer. The results show that it is necessary to maintain low and stable values for both concentration of the complex and its gradient, if it is expected to grow Cul single crystals with regular shape and large size. For the case of decreasing the concentration of feed solution gradually, the concentration of the complex and its gradient in the region of crystal growth were controlled in the range of 0.01-0.03 mol/l and at about 0.012 mot/(1 cm), respectively, so that the supersaturation of CuI (.) HI complex could keep a value which could make the Cul crystals growing gradually. In order to control the variation of the concentration of CuI (.) HI complex in silica gel, the diffusion rule of the complex was studied by increasing the concentration of feed solution gradually. The average diffusion coefficient was measured to be 1.521 x 10(-5) cm(2)/S. The results could provide a useful clue to further optimize the experimental scheme. (c) 2006 Elsevier B.V. All rights reserved.