This paper describes a system for controlling the introduction of purge fuel vapors from the canister purge vapor recovery system into an internal combustion engine, and for correcting the engine fueling due to the additional purge fuel vapors. The system utilizes a gasoline vapor concentration sensor to measure the concentration of fuel vapors purged into the engine, and a compensation algorithm to calculate a desired flow of purge fuel vapors into the engine. Vehicle testing on a vehicle equipped with a four-cylinder engine and the closed loop purge control system described in this paper has shown that significant reductions in tailpipe emissions may be achieved. Compared to the baseline vehicle, the control algorithm using the gasoline vapor sensor as a feedback signal reduced the emissions by 25% for HC, 38 % for CO, and 18 % for NOx. Vehicle testing has also demonstrated that purge aggressiveness may be substantially increased, i.e., the ability to purge larger quantities of fuel vapor was improved.