Hydrogen was used to operate a single cylinder engine in homogeneous charge compression ignition (HCCI) mode. The engine was a modified 435 cm3 single cylinder air cooled Yanmar L100V direct injection (DI) compression ignition (CI) engine. The original diesel fuel injection system was removed and a hydrogen port fuel injection (PFI) system was added, along with a 1 kW intake air heater. The piston was modified from the original re-entrant bowl piston to a dish shaped piston, while maintaining the original 21.2:1 compression ratio. The engine speed was maintained at a constant 1800 RPM. Three hydrogen fueling conditions of 25, 30, and 35 slpm were investigated, which corresponded to an excess air ratio (λ) of roughly 4.38, 3.64, and 3.16, respectively The fuel conversion efficiency for the conditions tested ranged from 23% - 27%. The low fuel conversion efficiency was partly attributed to the low combustion efficiency seen, which ranged from 86% - 88%, and partly attributed to the high fraction of energy leaving as heat transfer to the cylinder walls. It is speculated that the high fraction of fuel energy leaving as heat transfer to the cylinder walls is caused by the elevated intake air temperatures leading to higher in-cylinder temperatures during the compression stroke, the high surface area to volume ratio of the engine, and the operation of the engine at light loads.