This Bureau of Mines report evaluates the performance of commercially available portable instrumentation for the detection of carbon monoxide, carbon dioxide, oxygen, and methane at conditions simulating an industrial work atmosphere. Portable instrumentation included in the study was gas-detection tubes (length-of-stain and color intensity), catalytic detector, liquid absorption detectors, paramagnetic oxygen detector, galvanic detector, and "pelement" or hot-wire methane detectors. Evaluation of the instruments was in atmospheres with 50 and 80 percent relative humidities at 75 deg f and containing test gas concentrations of 0.1, 0.25, 0.50, 1.0, 1.5, and 2.0 Times the threshold limit value. Correlation of the results from the portable gas analyzer with measurement from the standard instruments was made by calculating and plotting an estimated line of regression, prediction, and confidence limits and the correlation coefficient for each detector system. Relative humidity in the 50- to 80-percent range did not appear to contribute to instrumental uncertainties in either performance or operation. Considerable deviation in detection precision between the portable gas-detection systems and the standard systems was established, especially at the lower gas concentration levels.