This study presents the first investigations on the micro-mixing properties of pharmaceutical powder blends from a resonant acoustic mixer using near-infrared chemical imaging. All experiments were done in a laboratory resonant acoustic mixer (RAM). The powder blends were studied using near-infrared chemical imaging (NIR-CI). Qualitative (i.e. chemical images) and quantitative (e.g. mean diameter of aggregates) results were obtained using this analytical method. The quantitative results were correlated to the acceleration (mixing intensity) and total mixing time. Overall, the resonant acoustic mixing performance increased with increasing acceleration and mixing time. Therefore, larger aggregates of the active pharmaceutical ingredient (API) were found at lower accelerations (mixing intensity) and shorter mixing times. Mixing in the RAM efficiently reduced the overall aggregate size of the cohesive API (semi-fine APAP, similar to 45 mu m) used in a common blend of filler (microcrystalline cellulose, similar to 110 mu m) and lubricant (magnesium stearate, similar to 10 mu m). (c) 2016 Elsevier B.V. All rights reserved.