Drop momentum and drop slip in the transient gas-phase environment found in the intake port of a spark ignition engine play a role in the fuel delivery process to the cylinder A study was conducted to investigate drop dynamics using a single-cylinder port fuel-injected engine fitted with a two-dimensional optically accessible intake port. The experimental procedure used externally clocked single-component phase Doppler interferometry which provided time-resolved results with respect to the engine cycle. The effects of fuel drop size and valve timing on drop trajectory and entrainment into the gas-phase flow were examined The data were analyzed according to drop size and lime of arrival at the probe volume. It was found that entrainment of the liquid phase into the gas phase is a function of both drop size and the local rate of change of the gas-phase velocity vector. A temporal Stokes number analysis was developed and compared to the experimental results to provide quantitative predictions of drop size requirements for entrainment during gas-phase transients.