Concerns over the effects of combustion chamber deposits (CCD) on engine performance coupled with increasing environmental pressures on vehicle exhaust emissions, have forced the original equipment manufacturers (OEMs) to call for control of fuel compositions to preclude or minimise deposit formation in the combustion chamber. However, this is currently not felt to be feasible, given the scarcity of data on the effect of CCD on emissions, and the lack of an industry standard measurement technique. This paper describes a two-year project which demonstrates the positive impact that a fully synthetic CCD control additive system has on engine performance and emissions. The same programme demonstrates that tailpipe HC, CO, and NOx all directionally increased after the deposit accumulation period, but only NOx decreased with the removal of CCD. Engine-out NOx emissions increase with heavy CCD weight.