The CT values of a variety of materials were studied in an EMI and a Syntex head scanner. The presence of bone-simulating rings changed the CT values despite the use of constant length water paths and software corrections. Errors due to beam hardening in CT scanning are discussed. These errors could be of significance, particularly in quantitative studies. The changes in CT values with KV setting of the scanner are used to illustrate their energy dependence and the peculiarities of the scaling system introduced by EMI. The difficulty in specifying a standard scale or unit for CT scanners is discussed.