In a pilot-plant trickle-bed hydrotreating reactor, ideal operating regimes (plug flow, full catalyst wetting, and absence of reactor wall effects) are desirable to generate reliable, reproducible, and representative data for kinetics studies and commercial scale-up. In this study, the operating regimes of a heavy gas oil pilot-plant hydrotreating reactor were investigated under operating conditions of commercial interest using real petroleum feedstocks. Vapor liquid equilibrium experiments and calculations were performed to accurately predict the fluid flows and the physical/transport properties of the vapor and liquid phases. These properties were subsequently used to predict the operating regimes under various operating conditions and to generate a map of operating conditions within which the desired operating regimes can be maintained.