In this work, the transverse mixing of free flowing particles in a horizontal rotating drum has been studied. Special attention is given to the initial moments of mixing and the transition from a standstill to a fully developed rolling regime. The parameters studied include rotational frequency (1.3 to 4.6 rpm), loading (15 to 50%), relative black/white powder content (1/3 to 2/3) as well as drum diameter (50 to 75 mm). To this end, four methods have been used to quantify dispersion: (1) Discrete Wavelet Transform (DWT), (2) Gray-Level Co-occurrence Matrix (GLCM), (3) Contact and (4) Entropy. Of these four methods, the GLCM algorithm was found best suited for this application: it is fast, reliable and easily interpretable despite requiring little user input. While Wavelet and Contact analysis methods also provide good results, Entropy systematically overestimates the degree of mixing for any given time, The results show that mixing dynamics from standstill can be modeled by a second-order dynamic based on the filling ratio and the number of drum rotations. (c) 2007 Elsevier B.V. All rights reserved.