The operation of homogeneously charged compression-ignition (HCCI) engines at low loads often suffer from excessive cyclic variations since the homogeneous charge is too fuel-lean to burn completely. These variations can be further exacerbated if natural gas is used due to the relatively narrow operational limits of methane. The paper describes the results of an analytical approach that models in full the reaction rate development when hydrogen is added to a naturally aspirated, methane or propane-fuelled motored HCCI engine. The operating conditions at which these effects are studied are those that are conducive to the onset of cyclic variations, such as fuel-lean conditions and low volumetric efficiencies. It is shown that the addition of hydrogen, which has a relatively wide operating range, to a methane- or propane-fuelled engine can reduce cyclic variations while extending the operating region of the engine. (C) 2000 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.