Prior studies have shown that fuel addition during negative valve overlap (NVO) can both increase temperature and alter composition of the charge carried over to main HCCI combustion. Late NVO fuel injection, i.e., near top dead center, can cause piston wetting and subsequent localized rich flames. Since acetylene is a product of rich combustion and is known to advance ignition, it is hypothesized that the species could play a chemical role in enhancing main combustion. The objective of this work is to quantify the effects of acetylene on HCCI combustion.While the research topic is specifically relevant to NVO-fueled HCCI operation, the experiments are conducted without NVO fueling to avoid uncertainties of NVO reforming reactions. Instead, a single post-NVO injection of iso-octane fuels the cycle, and acetylene is seeded into the intake flow at varying concentrations to simulate a reformed product of NVO. Total chemical energy is held constant by adjusting injected mass of iso-octane to compensate for added acetylene. In addition, combustion phasing is held constant by adjusting intake temperatures. The temperatures required to achieve target phasing, as well as heat-release analysis, provide a measure of acetylene's enhancement of main combustion. Chemical kinetics simulations using a piston/cylinder reactor model add insight to the experimental results.Results show that few-parts-per-thousand concentrations of acetylene reduce charge temperatures that are required in order to achieve target combustion phasing, increase peak heat-release rates, and improve combustion efficiency.