X-ray microtomography has been used to study in situ the uniaxial tensile response of low-density polyurethane foam. Two variants have been examined, one before and one after treatment to generate auxetic behaviour. For both variants, microstructurally faithful finite element (FE) models have been constructed from the initial tomographs. For each variant a series of tomographs have been collected during progressive straining. Poisson's ratios of 0.30 (conventional, non-auxetic) and −0.22 (auxetic) have been measured for the two variants by digital image correlation (DIC) between successive images. By comparison, the FE models exhibited Poisson's ratio's of 0.5 and −0.3, respectively. Key micromechanical mechanisms responsible for the auxetic effect have been observed during straining, such as the straightening of bent ribs and rotation of nodes (joints), compared to changes in the angles between essentially straight struts for the non-auxetic variant. The microstructurally faithful FE models confirm the mechanisms observed in the experiments and enable characteristic rib and node behaviour to be followed in greater detail.