The gas transfer efficiency and the peak height response have been studied as a function of acceptor and donor flow-rates for three different membrane types in flow-injection manifolds for determination of ammonia nitrogen. The common PTFE tape has a transfer efficiency of Z-17% at a sampling frequency of 60 h-' and about Z-23% at a sampling frequency of 30 h-I An increase in acceptor flow-rate decreases the peak height response considerably and more than the corresponding increase in donor flow-rate both for concurrent and countercurrent flow arrangements in the gas diffusion cell. Celgard 2400, a polypropylene membrane, gives about one third of the efficiency and the response obtained with PTFE tape. With respect to peak height response the most important variables are: injection volume, groove length in the gas diffusion cell, sodium hydroxide concentration, and membrane type. By joining the waste streams or by placing the manifold in an ultrasonic bath some increase in peak height response can also be accomplished. A non-uniformly performed installation of certain PTFE membrane types in the gas diffusion cell may change the response level by as much as 19%. An installation pressure of 4.5 Nm in flow-injection gas diffusion cells is recommended.