This paper investigates the starting transients in vacuum ejector and its dependence on various geometric configurations. Various geometric parameters investigated were the ratio of diffuser to primary duct height (D∕d), diffuser length–to–diffuser height ratio (L∕D), and secondary chamber height–to–secondary jet thickness ratio (hs∕ts). It is seen that the vacuum ejector start-up is associated with a gradual initial vacuum generation stage, which is followed by a rapid vacuum generation. The gradual vacuum generation is found to be caused by the mass supply by the recirculation bubble (existing in the diffuser) to the secondary chamber exit plane. The rapid evacuation happens when the initial recirculation bubble splits up, which results in a rapid reduction of the amount of reverse flow mass reaching the secondary chamber. It is found that with reduction in the D∕d ratio and increase in the L∕D ratio, the rapid evacuation stage reduces and eventually vanishes. The present investigation shows that both these configuration changes reduce the mass supply by reverse flow to the secondary chamber and this is attributed to the cause of the reduction in rapid evacuation stage. The present study also shows that the nature of vacuum generation does not vary with change in secondary chamber height. © 2019 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.