Lanthanide‐based compounds such as Gd‐DTPA are currently used as contrast agents in MRI. Recent experiments using CT and transmission radiography show that Gd agents can increase image contrast by up to a factor of 2 relative to more commonly used iodinated agents on an equi‐molar basis. It has also been suggested that beam hardening artifacts may be reduced with Gd. This hypothesis was experimentally tested on three different CT scanners using a circular water equivalent phantom with contrast filled tube inserts. It was found that the artifacts were a factor of 1.3–1.8 more pronounced with the iodinated contrast compared with Gd‐DTPA. A theoretical model which uses an experimentally derived photon energy spectrum is proposed which relates the strength of beam hardening artifacts to the variance (over the energy spectrum) of the attenuation coefficient of the contrast agent. This allows easy assessment of the relative magnitudes of the artifact for different contrast agents.