Liquid-to-air membrane energy exchangers (LAMEEs) use semi-permeable membranes to transfer heat and moisture between air and desiccant solution, and prevent the transfer of desiccant droplets to the air side. A 2-fluid flat-plate LAMEE is composed of several adjacent air and desiccant solution channels each separated by a semi-permeable membrane. A 3-fluid LAMEE is the new generation of LAMEEs which has a structure similar to a 2-fluid flat-plate LAMEE. The novelty of the 3-fluid LAMEE is that it includes titanium tubes inside the desiccant solution channels to control the temperature of the desiccant solution along the exchanger. In this paper, the performances of a 2-fluid LAMEE and 3-fluid LAMEE are tested and compared under diluted desiccant solution regeneration operating conditions. Also studied are the effects of operating conditions (inlet heating water temperature, heating water mass flow rate, desiccant solution mass flow rate, and inlet desiccant solution temperature) on the performances of the 2-fluid and 3-fluid LAMEEs. For the chosen test conditions, results show that effectiveness and moisture removal rate of the 3-fluid LAMEE are higher than the 2-fluid LAMEE under the entire range of inlet heating water temperature, heating water flow rate, and desiccant solution flow rate studied. The effectiveness and moisture removal rate of the 3-fluid LAMEE increase as the inlet heating water temperature and/or flow rate increases. The sensible, latent, and total effectivenesses, and moisture removal rate of the LAMEE increase by 38%, 40%, 39%, and 6 times when the temperature of desiccant solution is maintained constant along the exchanger. Compared with the 2-fluid LAMEE, the sensible, latent, and total effectivenesses, and moisture removal rate of the 3-fluid LAMEE are improved by up to 104%, 141%, 128%, and 17 times, respectively. (C) 2015 Elsevier Ltd. All rights reserved.