Mixtures of hydrogen and carbon monoxide were used to simulate the fuel component of synthesis gas and operate a single cylinder engine in homogeneous charge compression ignition (HCCI) mode. The engine was originally an air-cooled direct injection (DI) compression ignition (CI) engine. The original diesel fuel injection system was removed and a port fuel injection (PFI) system with intake air heating was added. The engine speed was maintained at a constant 1800 RPM. Three synthesis gas fuel compositions were tested, which comprised of 100% H-2, 75/25 H-2/CO ratio, and 50/50 H-2/CO ratio, by volume. These compositions were investigated at an equivalence ratio (EQR) of 0.26 and 0.30. In-cylinder pressure and H-2/CO emissions measurements were made at all conditions. To achieve peak indicated mean effective pressure (IMEP) at a given equivalence ratio, the intake air temperature had to be increased with increasing CO fraction in the synthesis gas mixture. For the EQR = 0.26 conditions, the intake air temperature required for best IMEP was 78 degrees C, 84 degrees C, and 98 degrees C, for 100% H-2, 75/25 H-2/CO ratio, and 50/50 H-2/CO ratio, respectively. For the EQR = 0.30 conditions, the intake air temperature requirements were 62 degrees C, 71 degrees C, and 81 degrees C, for the same respective H-2/CO proportions. The peak in-cylinder temperatures for all conditions tested ranged from roughly 1200 K-1500 K depending on intake air temperature, mixture concentration, and fuel composition. The combustion event was short, with the rapid burn angle ranging from 9.5 CAD to 11.5 CAD for all conditions tested, and the synthesis gas mixture composition did not change this significantly. The combustion efficiency was between 83% and 88% for the peak IMEP conditions tested. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.