Carbon fibers posses a unique set of physical, electrical, and thermal properties. Until recently these properties have not been exploited to the fullest due to the expense of the fibers. Conoco, Inc. is currently bringing to market a mesophase pitch-based carbon fiber that is made from the byproducts of the oil refining process. This technique promises a new carbon fiber with excellent properties at a lower cost. An understanding of the mass transfer and thermodynamic properties of solvents in the pre-carbonized mesophase pitch is essential to the quality control of the product. A diffusion study is being conducted on solvent interactions with an anthracene-based pitch. An anomalous solvent trapping effect has been observed in these systems. It is shown that only a certain amount of phenanthrene (solvent) is desorbed from the pitch into a pure stream of flowing nitrogen at a set temperature. The residual amount of solvent left in the pitch and the diffusion coefficients calculated for the mobile portion of the solvent are reported.