The NIOSH investigation of a commercial airborne vapor monitor (AVM) used for detecting organophosphorus pesticides was described. The AVM was a portable ion mobility spectrometer which could be used continuously for at least 6 hours. Data obtained by the AVM could be displayed as a simple bar graph of intensities; when used in conjunction with an IBM compatible computer, it could provide information on the concentrations of the measured species. The AVM was used in a field study of diazinon (333415) exposures in greenhouses. Its data were compared with the results of the analysis of air samples using NIOSH Method 5600. When tested with pure samples of diazinon, malathion (121755), methylparathion (298000), and other organophosphorus pesticides, all compounds produced a peak on the computer screen. Some of the compounds produced multiple peaks with similar drift times. In the greenhouses, the AVM did not produce a response on the bar graph display when microencapsulated diazinon formulations producing air diazinon concentrations of 13 to 28 micrograms per cubic meter (microg/m3) were being used. Many false positive responses were also seen. These were caused by xylene, the solvent in the diazinon formulation, cleaning agents, and fertilizers in the greenhouses. When used with the computer interface, AVM could not detect diazinon during cold fogging operations due to its detector being contaminated by high aerosol concentrations. The AVM detected diazinon at concentrations of 5 to 10microg/m3 in the ambient air of greenhouses after the pesticide had been sprayed. It was unable to detect surface diazinon concentrations on contaminated gloves or surfaces, possibly due to rapid adsorption by the surfaces. The author concludes that the AVM is not suitable for evaluating exposures during cold fogging operations or from surface contamination. It can be used as an indicator of safe reentry after application of an organophosphorus pesticide.