Vapor pressure determination of agrochemicals is required for EPA and EC registration. The Knudsen effusion method is an approved regulatory method of performing vapor pressure measurements. The Knudsen effusion method is a dynamic technique based on the rate of escape of vapor molecules through an orifice into a vacuum. We have modified the Knudsen effusion apparatus, replacing a conventional microbalance with a quartz crystal microbalance (QCM). The QCM has a sensitivity of less than 1 ng; therefore, small mass rate losses can be observed, reducing experimental time. The use of the QCM allows rapid determination of vapor pressures as low as 10 mu Pa. The vapor pressure of several agrochemicals has been measured using this modified method, and the results agree with previously reported values. The results and methodology are discussed.