A novel technique to determine in situ the free expansion kinetics of chemically foamed thermoplastic foams, so-called optical expandometry, is presented in this work. This technique is based on the camera monitoring of a free-foaming material placed inside a furnace. Images are acquired under special illumination conditions to facilitate the later image processing. The present article explains the experimental set-up and the image processing methods used to determine the free volumetric expansion of different polyolefinbased foams. The results are compared with those obtained using thermo-mechanical analysis. In addition, several possible applications of this method are detailed, such as studying the effect of polymer rheology, foaming temperature, blowing agent content and the anisotropy of the expansion.