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A well-developed technique, based upon a combined experimental and phenomenological modeling approach, has been adopted for the measurement of nitric oxide (NO) emissions in a diesel engine exhaust under transient engine operation. The dynamic behavior of the chemiluminescent analyzer used for the measurement of NO emissions is characterized by a single gas diffusion model followed by three perfect gas-mixing models. The response of the analyzer is obtained using a computer-controlled transient NO emission simulator, which randomly generates a square-pulse train of NO gas and concurrently measures its signal output. Signal prediction and reconstruction analysis were performed, which has demonstrated the predictive and reconstructive capabilities of the phenomenological analyzer model. On-line nitric oxide emission measurements were obtained in a computer-controlled dynamic engine test bed having three different control modes and fuel injection timing settings. The results have shown that the transient nitric oxide concentration is very sensitive to the engine operating condition prior to the transients as well to the amount of fuel injected into the combustion chamber at the start of the transient operation.