The UBHC emissions during cold starting need to be controlled in order to meet the future stringent standards. This requires a better understanding of the characteristics of the time resolved UBHC signal measured by a high frequency FID and its phasing with respect to the valve events. The computer program supplied with the instrument and currently used to compute the phase shift has many uncertainties due to the unsteady nature of engine operation during starting. A new technique is developed to measure the in-situ phase shift of the UBHC signal under the transient thermodynamic and dynamic conditions of the engine. The UBHC concentration is measured at two locations in the exhaust manifold of one cylinder in a multicylinder port injected gasoline engine. The two locations are 77 mm apart. The downstream probe is positioned opposite to a solenoid-operated injector which delivers a gaseous jet of hydrocarbon-free nitrogen upon command. The time period between the start of nitrogen injection and the sudden drop in the UBHC signal is measured and considered to be equal to the phase shift of the UBHC signal. Tests are conducted at normal and subzero room temperatures. The phase shift is determined while the exhaust valve is opened and closed. A comparison is made between the measured phase shift and the computed results. The effect of errors in the phase shift on the characteristics of the UBHC signal is examined under steady and transient conditions.