In this paper, we reported an improved preparation of 1-methyl-3-propylimidazolium iodide (MPII), which was the alkylation reaction of n-propyl iodide and 1-methylimidazole under solvent-free conditions by Teflon-lined, stainless autoclaves. It was shown that the resulting MPII was high pure, the conversion rate of 1-methylimidazole was close to 100% and the procedure was simple and eco-friendly. Moreover, the apparent diffusion coefficients of triiodide and iodide in the mixture with different ratios of MPII and 3-methoxypropionitrile were demonstrated by cyclic voltammetry using a Pt ultramicroelectrode. The dye-sensitized solar cells with the electrolyte, which was composed of 0.13 M I-2, 0.10 M LiI, 0.50 M 4-tert-butylpyrdine in the mixture of 3-methoxypropionitrile and MPII (weight ratio 0.65:1), gave short circuit photocurrent density of 14.82 mA/cm(2), open-circuit voltage of 0.69 V, and fill factor of 0.66, corresponding to the photoelectric conversion efficiency of 6.73% at the illumination (AM 1.5, 100 mW/cm(2)). (C) 2008 Published by Elsevier Ltd.