Displacement processes of oil by water in lowpermeable reservoir chalk from the Danish part of the North Sea have been studied by use of xray CT-scanning. The chalk is characterized by its low permeability, Le, 0.1 to 10 mD, and its relatively high porosity, i.e. 20-45%. The influence of the following parameters has been investigated: initial saturations, viscosity. ratio, interfacial tension, and flow rate. The information obtained by x-ray CT-scanning, which is not available from an ordinary massbalance monitoring, inciuding: a) The influence of rock heterogeneity on initial fluid saturations and the displacement process. b) The dominating character of the capillaiy end effects in experiments with high interfacial tension. c) A correct interpretation of the resuits at low interfacial tension which is a combination of increased microscopic displacement efficiency and a reduction of the capillary end effect.