Desiccant materials are commonly used in the automotive industry to reduce the level of moisture in vehicle air conditioning systems. The primary purpose for removing moisture from these systems is to avoid corrosion of metals, compatibility problems with polymeric materials, and possible freeze-up associated with free water. In nonpolar R-12/mineral oil systems with low solubility for water, moisture levels are usually controlled to 25 ppm or less. However, R-134a and PAG are highly polar and have good solubility for moisture, thus presenting reduced risk of free water in the air-conditioning systems. This paper addresses the questions of whether desiccants are required in air conditioning systems using R- 134a/PAG, and if required, what is the optimum quantity of desiccant for system stability and long-term system reliabilityTests were conducted in the laboratory (accelerated sealed tube aging according to ASHRAE standard 97- 1989) as well as in the field (vehicle fleet tests). The experimental results showed that the moisture level in the aged or retrieved PAG is affected by the amount of desiccant present in the system. However, the higher moisture levels recorded with reduced amount of desiccant or with no desiccant did not necessarily lead to high total acid numbers indicative of PAG decomposition or high fluoride ion concentrations indicative of R-134a decomposition. The effects of high moisture levels on various material components of the automotive air-conditioning system are also discussed.