An optofluidic method based on a liquid-core and liquid-cladding optical waveguide is proposed for roughly estimating the extent of diffusive mixing between two fluids. The mutual diffusion coefficient was estimated from a charged coupled device (CCD) camera image of light-focusing in accordance with predictions of a three-dimensional model. To check the validity of our proposed method, mutual diffusion coefficients of aqueous solutions of ethylene glycol at different volume fractions were measured, firstly. Then, mutual diffusion coefficients of eight 1,3-methylimidazolium ionic liquids-water binary systems were estimated at 298.2 K. Our results show that the mutual diffusion coefficients, in order of decreasing values, are as follows: [Emim][N(CN)₂] > [Mmim][BF₄] > [Emim][BF₄] > [Bmim][NO₃] > [Bmim][N(CN)₂] > [Emim][ClO₄] > [Bmim][BF₄] > [Bmim][ClO₄], it is well correlated to molecular weights of the ionic liquids. Finally, mutual diffusion coefficients of three ionic liquids-water binary systems at temperatures ranging from 298.2 to 338.2 K were measured, which is sensitive to temperature and increases quickly with it. Our results show a moderate discrepancy, but this proposed optofluidic method allows one to perform diffusion measurements with a simple, costless and microlitre volume manner in highly confined spaces.