A split-detector system, consisting of a thin CaF2 scintillator followed by a thick Nal crystal, was installed in an EMI Mark I scanner. Energy discrimination is possible because the CaF2 crystal responds primarily to low-energy photons; dual-energy images can be obtained from only one scan. Calculations and preliminary experiments show that the split-detector separates energies better than the 100-140 kVp method. It was possibel with the split detector to differentiate between two weak solutions of CaCl2 and Kl which had the same computed tomography number (44 H), under normal scanning conditions.