<p>Heat carrier fluids are becoming increasingly important in modern energy systems, enabling improved efficiency and functionality in applications such as 5th generation district heating, district cooling, geothermal systems and data centre cooling. These fluids, often glycol- or ethanol-based, play a critical role in heat transfer processes, where the accurate determination of their specific heat capacity is essential. Specific heat capacity not only influences the design and operation of such systems but is also a key parameter for the reliable calculation of heat quantities, particularly in monetary billing. However, while the specific heat capacity of water can be measured with low uncertainty, the determination of this property for other heat carrier fluids often involves greater uncertainties. This paper presents the results of an interlaboratory comparison to evaluate the uncertainty with which the specific heat capacity of common heat carrier liquids can be measured under typical laboratory conditions. The findings aim to highlight the achievable uncertainty, identify potential sources of uncertainty, and provide guidance for improving measurement reliability in routine laboratory practice.</p>