A series of critical experiments with blocks of 2% U235-enriched UF4-C25H52 has been initiated at the ORNL Critical Experiments Facility. Thus far assemblies with H:U235 atomic ratios of 195 and 294 have been built in parallelepipedal and simulated cylindrical geometries, both reflected and unreflected. From the results the minimum critical masses for reflected spheres have been determined to be 16.3 and 8.5 kg of U235 for fuel mixtures with H:U235 atomic ratios of 195 and 294, respectively. The minimum critical masses for unreflected spheres of these two fuel mixtures are 24.3 and 12.7 kg of U235, respectively.