The 3M Formaldehyde Monitor and the DuPont Pro-Tek Formaldehyde Badge were evaluated for performance and reliability. This evaluation revealed that the 3M monitor results were variable and lower than reference concentrations determined independently. When the monitors were humidified before use and then exposed in humid (ca. 80% RH) formaldehyde-containing atmosphere, the monitors did give accurate results. Results of additional experiments led to the conclusion that quantitative reaction between formaldehyde generated in our chamber and the absorbent pad in the 3M monitor required the presence of a minimum level of absorbed water. The DuPont badges gave good agreement with the reference concentrations determined independently under the following conditions: sampling period of 1 to 12 hr with a minimum integrated sample loading of 4 ppm-hr; at least 3 m/min (10 ft/min) face velocity; correction for blank badges; and correction for loss of reagent from the samples. The evaluation indicated that the DuPont badge was not well suited to short term sampling and was subject to evaporation of liquid from the absorbing liquid blister. Blank values also were found to be variable, necessitating the analysis of several blanks to be used for blank correction with each set of samples. This blank variability also contributed to high variability found when short term measurements were made. The badge also had a negative interference from phenol at high phenol-to-formaldehyde ratios. Some of the major problems observed with both passive monitors were found only after devices which had been aged under storage conditions were analyzed and these results interpreted. If all testing had been done with fresh sampling devices, many of the problems would not have been noticed. Based on the results of this study, this factor of diffusive monitor aging needs to be addressed in any further work on passive monitor evaluation.