Computerized tomography (CT) is being increasingly applied to analyzing flow in porous media. Tomography systems being used include both medical x ray units and custom gamma ray machines. Up until now the precision of such systems in porous media work has been ill defined. A systematic analysis of CT performance and errors is presented and applied to a prototype gamma ray system we have recently developed. The prototype system is detailed and its accuracy quantified as a function of scanning beam diameter, ray step size, and photon count. ¿ American Geophysical Union 1993