Describes a DETR and industry-funded research project carried out at BSRIA to investigate alternative control strategies for existing air cooled chillers which incorporate thermostatic expansion valves and to consider ways of improving the chiller energy efficiency. Describes a test facility constructed to house a standard air-cooled chiller of 35 kW cooling capacity, which allowed the chiller load and ambient temperature to be controlled across a wide range of conditions. Presents and discusses the test results. Notes simple modifications to the "as supplied" control strategy that would produce energy savings over a full year. Notes further savings if the components that delivered head pressure control could be manufactured cost effectively to incorporate a slightly more sophisticated control strategy (proportional and integral action). Maximum savings would result from full condenser temperature control using microprocessor control with appropriate control algorithms. Notes potential CO2 savings. Provides recommendations for owners and operators of existing air cooled chillers and those installing new or replacement chillers. Also gives advice for designers and manufacturers.