Abstract A systematic experimental investigation of capillary pressure characteristics and fluid flow in diatomite has begun. Using an X-ray CT scanner and a specially constructed imbibition cell, we study spontaneous water imbibition processes in diatomite and for reference Berea sandstone and chalk. The mass of water imbibed as a function of time is also measured. Imbibition is restricted to cocurrent flow. Despite a marked difference in rock properties including permeability and porosity, we find similar trends in saturation profiles and weight gain versus time functions. Imbibition in diatomite is relatively rapid when initial water saturation is low due to large capillary forces.