Vehicle acceleration induced pressure spike on the high side of an Air Conditioning (AC) system is causing considerable concerns, especially for systems with high efficiency compressors. Head pressure surge in the order of one to two hundred pounds per square inch can be observed within a time span of 10 seconds or less. As the industry moves to meet increased system durability standards and passenger comfort requirements, clear understanding of the underlying mechanisms is required so that the impact of the head pressure spike can be minimized or eliminated.The present investigation seeks to understand the mechanisms of the head pressure spike phenomenon through both experimental and mathematical analyses. Experimentally, extensive testing has been conducted in environmental wind tunnel. Our mathematical analysis is based on the mass conservation principle for the refrigerant flow through the high side of an AC system. The analyses establish that the head pressure spike is primarily a phenomenon of mass balance between the refrigerant expansion device and the compressor. Consequently, all the variables in the AC system that have bearings over the mass flow rate through the compressor or the expansion device can all exert due influence on the magnitude of the head pressure spike. Based on the analysis, a number of practical measures are recommended for mitigating the head pressure spike, especially for scroll compressor AC systems.