Optical imaging diagnostics of combustion are most often performed in the visible spectral band, in part because camera technology is most mature in this region, but operating in the infrared (IR) provides a number of benefits. These benefits include access to emission lines of relevant chemical species (e.g. water, carbon dioxide, and carbon monoxide) and obviation of image intensifiers (avoiding reduced spatial resolution and increased cost). High-speed IR in-cylinder imaging and image processing were used to investigate the relationships between infrared images, quantitative image-derived metrics (e.g. location of the flame centroid), and measurements made with in-cylinder pressure transducers (e.g. coefficient of variation of mean effective pressure). A 9.7-liter, inline-six, natural-gas-fueled engine was modified to enable exhaust-gas recirculation (EGR) and provide borescopic optical access to one cylinder for two high-speed infrared cameras. A high-energy inductively coupled ignition system delivered 140 mJ of energy during each spark event. The engine was operated at 1000 rev/min and an indicated mean effective pressure of 6.8 bar over a range of air/fuel equivalence ratios, λ, (1 to 1.6) and EGR rates (2% to 23%). Strong emission lines of water are present in the sensitivity band of the cameras (1.0 to 1.7 μm) and can be used as a proxy for the flame front and burned-gas regions. Images were recorded every 5.5 degrees of crank angle (CAD); multiple measurements were interleaved to provide statistical information every 0.5 CAD. The greater cyclic variation resulting from lean/dilute operation is apparent in the images; the image-derived metrics measured early in the cycle correlate strongly with pressure-derived metrics measured later. Centroids calculated from the images show that flames farther from the head and spark plug yield better combustion, which is not evident in the pressure data.