In recent years, Homogeneous Charge Compression Ignition (HCCI) engines have received much interest, because they combine a high thermal efficiency with near-zero emissions of NOx and soot. However, the maximum attainable load is limited by the occurrence of ringing combustion. This happens when the combustion rate is too fast, leading to pressure oscillations in the combustion chamber accompanied by a knocking sound. Some researchers have hypothesized that these pressure oscillations increase the heat transfer from the combustion gases to the cylinder wall, due to the breakup of the thermal boundary layer. No experimental results are available to support this hypothesis, as no experimental studies have been conducted to investigate the heat transfer during ringing operation. The goal of the current work is to investigate how the occurrence of a ringing combustion alters the heat transfer and which physical phenomena are responsible for it, in order to propose the best modeling approach. The heat transfer is measured inside a CFR engine converted to HCCI operation, with a thermopile heat flux sensor. HCCI operation is obtained by using a low octane fuel (n-heptane) and by preheating the inlet air. A variation in mass fuel rate at different compression ratios is performed to measure the heat flux during three different conditions: no, light and severe ringing. The occurrence and the intensity of ringing are determined with the amplitude of the cylinder pressure oscillations and by calculating the ringing intensity. The measured heat flux is split up in the convection coefficient and the temperature difference between the gas and the wall. This decomposition makes it possible to determine which phenomena affect the heat flux. Furthermore, the heat flux is measured at the cylinder head and at the cylinder wall to investigate the spatial variation of the heat transfer.