Push-pull ventilation of open surface tanks is often the best and most energy efficient way to remove any contaminant which evaporates from the tank. Existing design guidelines which are usual based on experimental work, are contradictory, and cannot easily be extended to different operating conditions. Numerical approaches to the push-pull ventilation system are particularly time consuming because of the Geometry of the system. Therefore in this paper, we present a semi-analytical approach which exploits the self-similar nature of much of the fluid flow. From this we are able to establish the nature of both the fluid flow. and the transport of contaminant, and these results are compared with numerical solutions. From an understanding of the fluid flow, it follows that the most significant parameter in the system is the initial momentum of the jet, or 'push' flow. It will be shown that in most cases this should be proportional to the length of the tank. and more detailed guidelines are given for when the contaminant is buoyant. The exhaust flow rates can then be calculated based on the required jet parameters. Copyright (C) 2003 John Wiley Sons, Ltd.