Solidification of metal foams was studied by in-situ X-ray radioscopy. Precursors were prepared by uni-axial hot compaction of metal/blowing agent powder blends, foamed in a X-ray transparent furnace and were then solidified. Foam evolution was continuously monitored. The cross-section of the foam at any instant was determined from the images by automatised image analysis. During cooling of the sample an unexpected small transient expansion stage was observed. In order to explain this behaviour different possible effects are proposed and evaluated. Experiments were carried out for alloys with and without blowing agent, for different heating times and cooling rates to separate the various possible mechanisms. It was observed that the transient expansion strongly depends on heating time and cooling rate. Some transient expansion is also observed for alloys without blowing agent demonstrating that the effect can only be partially ascribed to the behaviour of the blowing agent.