This paper describes an experimental study comparing the peak ignition voltage requirements of natural gas and gasoline in a typical bi-fuel vehicle application. Chassis dynamometer tests were carried out in which the vehicle was subjected to different types of transient wide open throttle events to create “worst case” voltage requirements. In addition to measurements of ignition voltage, other factors known to influence voltage requirements (such as cylinder pressure, electrode temperature, and fuel/air ratio) were recorded during the transient tests in order to obtain a better understanding of the underlying reasons for observed differences in voltage requirements between the two fuels and between the different transient test procedures.The results presented in this paper quantify the increased peak voltage requirements (relative to gasoline) for reliable ignition of natural gas under various operating conditions. The effects of voltage polarity and spark plug wear are also addressed. Electrode temperature is shown to be the dominant factor influencing how different transient test procedures affect the peak voltage requirement of a given fuel.