The performance of long term, or time weighted average, detector tubes was studied. Draeger-CH-28121, Kitagawa-Sipin-TWA-500, and MSA-Model-465519 tubes were tested by exposure to carbon-monoxide (630080) (CO2) at 48.9 and 94.5 parts per million (ppm). Sulfur- dioxide (7446095) exposures tested Draeger-Model-67-28151, MSA-Model- 92623, and Kitagawa-Sipin TWA-503 tubes at concentrations of 4.20, 8.34, and 2.32ppm. The lowest concentration of each was tested at relative humidities of 50 and 90 percent and at dry, while other concentrations were tested at 50 percent relative humidity. Results were compared to known values of the concentrations. In sampling CO2 results were generally good, with high positive bias of MSA tubes at low humidities and high positive biases of Draeger and MSA at low concentrations. Length of exposure also led to positive biases for all tubes. Results for SO2 were fair to good, and were poorer than CO2 results. At high humidities results were not reliable. All types of pump proved acceptable. The author concludes that the long term detector tubes have no substantial negative biases when manufacturers' recommendations are followed. The tendency toward positive bias could lead to increased protection for workers.