Hydrotreating has become a critical refining process as fuel sulfur specifications are tightened around the world. Recently, refiners in the United States have been learning how to optimize the performance of ultra low sulfur diesel (ulsd) hydrotreaters. The gas-to-oil feed rate ratio is known to be an important variable in this respect. It is well known that the gas-to-oil rate must be kept high enough to maintain the desired hydrogen partial pressure through the hydrotreating reactor, and to minimize the inhibiting effect of hydrogen sulfide. A lesser-known effect is the effect of gas-to-oil rate on the vapor-liquid equilibrium in the reactor. Changing the gas-to-oil rate alters the distribution of reactants between vapor and liquid in a way that changes the relative reaction rates of different sulfur compounds. This paper presents some pilot plant data and analysis showing this effect of phase equilibrium in deep diesel desulfurization. The effect can be modeled using the Frye-Mosby equation, which accounts for the effects of feed vaporization and phase equilibrium on the reaction rates of individual sulfur compounds in a trickle bed hydrotreater. (C) 2007 Elsevier B.V. All rights reserved.