The impact of mass extractions and injections as a method for increasing the energetic performance of fixed-size humidification-dehumidification desalination systems is examined. Whereas previous studies of this problem have been restricted to thermodynamic models, the use of a more complete model that includes transport provides the ability to quantify the impact of mass extractions/injections on a realizable, fixed-size system. For a closed air, open water cycle, the results show that a single water extraction from the dehumidifier to the humidifier increases the gained output ratio by up to 10%, with extractions higher in the cycle proving more effective. The sizing problem for the humidifier and dehumidifier under thermodynamically optimized conditions found in literature is also discussed, as is the impact of system size on overall performance of a system without extractions/injections. For a range of sizes, it is shown that a rough doubling of both dehumidifier and humidifier size results in a two- to three-fold increase in gained output ratio, with diminishing returns as the absolute sizes increase. (C) 2013 Elsevier B.V. All rights reserved.