A field trial in which outdoor air was sampled with an active reference sampler and several passive HTO-in-air samplers simultaneously was carried out at Chalk River Laboratories. Both passive and active samplers were changed on an approximately monthly schedule from 1990 September 2 to 1991 April 18. Average temperatures for the sampling intervals ranged from −8.6°C to +15.5°C and HTO-in-air concentrations measured by the active sampler were typically 10 Bq/m3. A total of 129 passive HTO-in-air sampler measurements were made during the seven sampling intervals. The passive samplers used for the field trial were of the type described by Stephenson,1 prepared with either tritium-free water or a solution of 50% tritium-free water and 50% ethylene glycol. As expected, the samplers prepared with the water-glycol solution performed more consistently than the samplers prepared with water only. Good agreement between passive and active sampler measurements was observed throughout the field trial. Passive samplers were found to be suitable for outdoor tritium-in-air sampling with a significant reduction in effort over traditional active samplers. However, passive samplers can not obtain the sensitivity of conventional active HTO-in-air samplers.