Recent investigations of the time dependence of electrical breakdown in liquid hydrocarbons have led to a disagreement regarding the influence of molecular structure upon the formative time lag. This paper represents an attempt to resolve the discrepancy, and to determine if possible the significance of the time lag in the breakdown process. The experiments involve the application of rectangular pulses of voltage of variable duration and amplitude to the liquids between hemispherical electrodes.The results of the investigation show definitely that the formative time lag is insensitive to changes in the molecular weight of the hydrocarbon liquid. Consequently, it cannot be associated with the time required for positive ions to cross the spark gap, as has been suggested by recent investigators. Its characteristic dependence upon the electrode separation, however, suggests that it may be mainly a measure of the transit time of an electron or an electron avalanche.