The compound 1-(4’-hydroxyphenyl)-2-phenylethane attached through an Si-O-Ar linkage to fumed silica was subjected to thermolysis (390-410 degrees C) under D-2 and N-2 (14 MPa). Product distributions under D-2 are similar to those previously observed in vacuum except for increases in yields of benzene and phenol expected as a consequence of hydrocracking. The high pressure of D-2 does not inhibit retrogressive reactions (rearrangement, cyclization) previously found to be promoted by restricted mass transport conditions. A demonstrated silica-promoted H-D exchange of small amounts of phenolic compounds present in equilibrium with surface-attached materials under the closed, high-pressure reactor conditions required for D-2 experiments complicates interpretation of deuterium distribution information. However, non-hydroxylated aromatic rings are immune to this process and measurement of their D incorporation shows that the normal D-atom-promoted exchange is more efficient in the gas phase than on the silica surface.