Several analytical methods were applied to the 3-D electrical resistance tomography (ERT) image of stirred-vessel experiments of Holden et al. to characterize the mixing behavior and to measure mixing quantitatively. Based on the time resolved 3-D ERT results, the centroid and variance of the concentration distribution in three dimensions can be obtained as a time series to track the movement, dispersion, stretch and rotation of the concentration distribution in the tank. By sampling at different scales and locations, the ratios of standard deviations to means and the entropy of corresponding volumes can be calculated to examine the degrees of uniformity. The 3-D wavelet decomposition provides an effective method of analyzing the mixing process at various scales. The wavelet difference coefficients reflect the concentration gradients of specific volumes along different directions. These methods of analysis promise to guide the design of better mixing schemes and to monitor the process for optimal operation.