In this study, an HCCI engine equipped with fully-flexible valve actuation was used to explore the effects of diluent composition on the rates of HCCI and spark assisted compression ignition (SACI) combustion. The diluent composition of the charge was characterized by the molar O-2 fraction in the unburned mixture which was modified at constant charge energy content through careful control of the exhaust gas recirculation (EGR) rate. During SACI operation the amount of EGR was systematically increased (thereby decreasing the molar O-2 fraction in the charge) and the temperature of the unburned gas was varied to maintain constant combustion phasing at constant spark advance. A similar to 45% relative increase in the molar O-2 fraction of the unburned mixture required a similar to 30 K decrease in the unburned gas temperature at intake valve closing (IVC) to maintain combustion phasing. Higher rates of heat release attributable to flames were observed as mixture O-2 fraction increased; however, under both HCCI and SACI conditions, the auto-ignition durations were relatively insensitive to diluent composition. As the charge molar O-2 fraction increased, the net indicated thermal efficiency of SACI combustion increased by 4% (relative basis) and peak rates of heat release decreased by similar to 24%. However, engine-out NOx emissions during SACI operation nearly quadrupled (260% increase) for the fuel lean mixture (highest molar O-2 fraction) compared to the stoichiometric EGR dilute mixture (lowest molar O-2 fraction). Two-zone heat release analysis was used to better understand the contributions of flame propagation and auto-ignition to the overall SACI combustion rate. (C) 2014 Elsevier Ltd. All rights reserved.