Prudhoe Bay field, the largest oil and gas accumulation in North America, has a large gas cap in communication with the oil rim. In addition to the liquid condensate held in the gas phase, the original gas cap contains relict oil that exists in the liquid phase at low saturations and is remnant of the oil migration - spill history of the reservoir. One of the key recovery mechanisms at the Prudhoe Bay field is the use of lean gas injection to increase recovery through vaporization of relict oil and dropped-out condensate in the original gas cap, and residual oil in the expanded gas cap. The lean gas vaporization project was implemented with an initial installed gas handling capacity of 2 BCF/D which was successively expanded to 8 BCF/D in several Gas Handling eXpansion (GHX) projects making it the largest hydrocarbon gas injection project in the world. After processing the produced gas to manufacture NGLs and Miscible Injectant, the lean gas is reinjected into the Prudhoe Bay gas cap for pressure support and vaporization of liquid phase hydrocarbons. In addition to estimates of black oil benefits, design and commercial evaluation of each stage of the gas handling expansion required accurate prediction of vaporization benefits from lean gas injection. A compositional surveillance program was designed and implemented to assess the efficiency of the vaporization process in the field. This paper describes the surveillance program in which the routine separator tests conducted at the specified well locations for fluid allocation and production optimization are modified to include additional measurements of extended separator gas compositions and separator oil API gravity. The paper also presents details of an analytical procedure in which the measured compositional information from the well tests is used together with other relevant field data to deduce the effectiveness of the vaporization process. The compositional surveillance data analysis procedure is demonstrated by application to the gravity drainage area of the Prudhoe Bay field. Deliberate application of the compositional surveillance data acquisition and analysis by the Prudhoe Bay Reservoir Management team will significantly impact the optimization of the vaporization process in the field.