The mixing of granular materials has a significant influence on the yield and/or quality of the desired products in numerous industrial processes including energy generation, food processing, and pharmaceutical production. However, characterizing the mixing effectiveness of systems or processes in granular applications is difficult due to challenging sampling procedures and measurement techniques. In this study, a two-part measurement technique consisting of optical visualization and composition analysis is developed to provide qualitative and quantitative mixing characteristics of continuous granular mixing processes, respectively. Mixing studies are performed in a laboratory-scale double screw mixer using a binary mixture of 500-6350 mu m red oak chips and 300-500 pm glass beads. The effect of screw rotation speed and dimensionless screw pitch on the mixing effectiveness is investigated for omega = 20, 40, and 60 rpm and p/D = 0.75, 1.25, and 1.75, respectively. Optical visualization in terms of video capture is captured across the entire mixing region's periphery, providing extensive qualitative observations. Quantitative composition analysis is performed on samples collected across the screw mixer and a two-way analysis of variance (ANOVA) statistical model is applied. Overall, the mixing effectiveness is maximized at an intermediate screw rotation speed of omega = 40 rpm and a dimensionless screw pitch of p/D = 1.75. The developed measurement techniques and resulting trends are compared to previous granular mixing studies featuring similar mixing equipment found in the literature. (C) 2014 Elsevier B.V. All rights reserved.