A method of conditioning a process stream of air in an air conditioning system wherein a process stream of air is dehumidified and cooled to provide a conditioned stream of air for introducing to a conditioned space. The method comprises the steps of providing an adsorption wheel having a multiplicity of passages through which process air can flow for adsorbing moisture therefrom, the wheel capable of adsorption of moisture from the process air and of regeneration on a continuous basis as the wheel rotates. An indirect evaporative cooler is provided having a dry side and a wet side separated by a moisture-inpervious wall wherein heat is extracted from said dry side through the wall to the wet side. Cooling in the dry side is achieved by evaporation of water into air passing through the wet side. The process air is passed through the adsorption wheel to remove moisture therefrom to provide a moisture-depleted stream of process air exiting the adsorption wheel. The adsorption wheel is regenerated by passing hot gases therethrough to remove moisture from the adsorption wheel. The moisturedepleted stream of process air exiting said adsorption wheel is divided into a relatively hot stream and a relatively cool stream, and the relatively hot stream of process air is introduced into the wet side of the indirect evaporative cooler, and the relatively cool stream is introduced into the dry side, the relatively hot stream evaporating water thereinto thereby cooling the moisture-impervious wall and removing heat from the relatively cool stream to provide cooled air to be introduced to a conditioned space.