The determination of energy expenditure (EE) is central to many applications within exercise physiology. The quantification of EE is usually carried out using indirect calorimetry. This relies upon the accurate measurement of VO2 and VCO2 to determine the oxygen cost of the activity and the substrate oxidised. EE is calculated using the formula: Total EE (kJ/min)=kilojoules per litre O2 consumed×VO2, where kilojoules per litre of O2 consumed is calculated from the thermal equivalents of oxygen for nonprotein respiratory quotient (RQ). However, there is variation in the specific thermal equivalents used. The impact that this has on estimated EE has yet to be described. The authors examined nine different methods that were grouped into three different categories. Six methods (C1) were based on the thermal equivalents tables of Lusk.1 Another category (C2) consisted of a major revision of the Lusk table.2 The final two methods (C3) used fixed values for the thermal equivalents of O2.3 The different categories for calculating EE were applied to data with a fixed VO2 of 2.5 litre/min and RQ values between 0.71 and 1. Differences between categories were examined using a one-way analysis of variance. The EE values within C1 showed a variation of less than 1.1% at an RQ of 0.85 (p>0.05). The EE based on the Peronnet et al table (C2) of nonprotein RQ differed by 3.2% from C1 (p≤0.01). The fixed methods (C3) showed a difference of up to 6.3% compared with C1 (p≤0.01). Variation in thermal equivalents values has a large impact on calculated EE. Fixed methods are overly simplistic as they do not take into account the substrate being oxidised. The figures of Peronnet et al (1991) are preferable to those based upon Lusk's early work as they are based on more accurate values of energy released from fat and carbohydrates.