The efficiency of positive-displacement components is of prime importance in determining the overall performance of a variety of thermodynamic systems. Losses due to the unsteady thermal-energy exchange between the working fluid and the solid walls of the device are an important loss mechanism. In this work, heat transfer in gas-spring devices is investigated numerically in order to focus explicitly on these thermodynamic losses. The specific aim of the study is to investigate the behaviour of real gases in gas springs and compare this to that of ideal gases in order to understand the impact of real-gas effects on the thermally induced losses in reciprocating expanders and compressors. This work relates these losses to the fluid properties and quantifies the influence of the thermophysical models applied. A CFD-model of a gas spring is developed in OpenFOAM. Four different fluid models are compared: (i) a perfect-gas model (i.e., an ideal-gas model with constant thermodynamic and transport properties); (ii) an ideal-gas model with temperature-dependent properties; (iii) a real-gas model using the Peng-Robinson equation-of-state with temperature and density-dependent properties; and (iv) a real-gas model using gas-property tables to interpolate values of thermodynamic and transport properties as functions of temperature and pressure. Results indicate that for simple, mono- and diatomic gases, like helium or nitrogen, there is a negligible difference in the pressure and temperature oscillations over a cycle between the ideal and real-gas models. However, when considering heavier (organic) molecules, such as propane, the ideal-gas model tends to overestimate the temperature and pressure (by as much as 20%) compared to the real-gas model. A real-gas model that uses the Peng-Robinson equation of state underestimates the pressure relative to the more accurate model based on look-up tables by as much as 10%. Furthermore, both ideal-gas and Peng-Robinson models underestimate the thermally induced loss compared to the table-based model for heavier gases. Different alkanes and alkane mixtures are also compared. It is found that, for a fixed volume ratio, pure and mixed alkanes that exhibit a higher heat capacity incur lower losses due to the lower temperature amplitudes, and thus, lower heat transfer occurring in the gas spring. For example, propane, which has a heat capacity only half of hexane, exhibits a loss of 5.1% (defined as the ratio of the net cyclic heat loss to the compression work), while the loss with hexane amounts to 3.6% (in both cases for a volume ratio of 6.63). Real-gas effects play an increasing role for heavier alkanes because the critical temperature and pressure are lower. The thermodynamic state of the gas is close to the critical point where real-gas effects are very prevalent. Finally, mixtures exhibit losses which lie between the value of their respective pure fluids, whereby increasing the proportion of the pure substance with the higher loss also leads to a higher loss for the mixture.