For the coronal mass ejection (CME) of January 6, 1997, strong element fractionation of the heavy ions was observed at 1 AU with the Mass Time-of-Flight (MTOF) sensor of the Charge, Element, and Isotope Analysis System (CELIAS) on the Solar and Heliospheric Observatory (SOHO). During the passage of the CME plasma and the passage of the erupted filament, which followed the CME, a mass-dependent element fractionation was found with an enhancement of heavy elements, increasing monotonically with atomic mass. Si/O and Fe/O ratios around 0.5 were observed, which corresponds to an increase of about a factor of 4 compared to regular slow solar wind. We present a theoretical model with which we can reproduce the observed element fractionation. The model assumes hot coronal loops with non-Maxwellian electron distributions as the precursor structure of the CME on the solar surface. Diffusion perpendicular to the magnetic field results in the preferential loss of lighter ions from the loop, leading to mass fractionation. To quantitatively reproduce the fractionation process, the loops must have existed for ~28 hours before they became part of the CME plasma, a time that is commensurate with optical observations of loops in the active region from which the CME was launched. ¿ 2000 American Geophysical Union