This paper presents the optimal design of a hybrid liquid desiccant-regenerative, evaporative air conditioner. A hybrid air conditioner was proposed which comprises a liquid-to-air membrane energy exchanger (LAMEE) as a desiccant dehumidifier and a regenerative heat and mass exchanger (RHMX) as an evaporative cooler. The air conditions across the LAMEE and the RHMX were achieved by two heat and mass transfer sub-models, and validation of the sub-models was conducted by comparison of the simulation results against the published experimental measurements. Intake air flow rate, LAMEE-tototal length ratio and working-to-intake air flow ratio are regarded as design variables. The simplified conjugate gradient method (SCGM) was employed as an optimization scheme to find the optimum set of design variables, which maximizes the room cooling capacity (RCC) of the proposed hybrid air conditioner. The influences of different design and operating conditions on the optimal RCC and the optimum set of design variables were studied. (C) 2016 Elsevier B.V. All rights reserved.