Theoretical thermal efficiencies and theoretical effectivenesses of six kinds of basic gas cycles, including the Carnot, Otto, Diesel, Takemura, Atkinson and Joule cycles, were compared with one another under the restriction of constant heat-addition. Relations between the available work content of heat added and the exergy change in several thermodynamic processes were also examined. It is concluded that the Atkinson cycle has the highest thermal efficiency and the second best effectiveness among the gas cycles examined. The exergy changes in an isochoric heating-process in closed systems and in an isobaric heating-process in open systems agree with the available work contents of heat added in these processes. Thus, the exergy increment in a heating process should not be generally adopted as the denominator for the theoretical effectiveness. (C) 2005 Elsevier Ltd. All rights reserved.