Presence of soot in gas analysis systems is believed to cause errors in measurement of the concentration of oxides of nitrogen (NOx) because of adsorption of NOx by soot. The work reported herein involves a preliminary experimental and theoretical analysis to determine the extent of the errors caused by the interaction of NOx with soot suspended in diesel exhaust measurement systems. The experimental results, which are in qualitative agreement with the theory, suggest that at room temperature the percentage of NOx adsorbed by soot suspended in the exhaust Stream is a function of the residence time of the gas sample in the sample line. Therefore, a short sample line and a high flow rate would minimize the error in the measurement of NOx. Application of the model to a production passenger-car diesel engine tested on the Federal Test Procedure and using the standard Constant Volume Sampling system indicates that the error in NOx measurement attributable to adsorption by soot is negligible.