An experimental study was conducted to quantify the effect of return air leakage from hot/humid attic spaces on the performance of a residential air conditioner Tests were conducted in psychrometric facilities where temperatures and humidities could be controlled closely. The test air conditioner had a nominal cooling capacity of 12.3 kW and a seasonal coefficient of performance of 3.8. Return air leakage from hot attic spaces was simulated by assuming adiabatic mixing of the indoor air at normal conditions with the attic air at high temperatures. Effective capacity and coefficient of performance both decreased with increased return air leakage, leakage air temperature, and air humidity. Under attic conditions of 54.4degreesC and 20 percent relative humidity, 10% return leakage reduced the effective cooling capacity and coefficient of performance of the air conditioner by approximately 30%. Power consumption was relatively constant for all variables except outdoor temperature. The sensible heat ratio (SHR), which is a measure of the dehumidification performance, increased with increasing leakage.