Many industrial processes (i.e., concentrated solar power [CSP] plants) require indirect heating of the product to temperature above ambient temperature. A heat transfer fluid (HTF), such as a mineral-based fluid like GlobalthermTM M (Global Heat Transfer; Staffordshire, UK), is used in such plants and flows from the heater to source requiring the heat. Not all HTFs are the same, however, and so understanding the difference between fluids is important to ensure that end-users use the correct fluid for the correct operation. This is especially true in CSP plants where HTFs operate at high temperatures for long periods of time and therefore need to be stable under such conditions. Indeed, biphenyl diphenyl oxide (BDO) mixtures are commonly used in CSP plants as they can be heated to 400 degrees Celsius, which is higher than the upper operating temperature for a mineral-based HTF (i.e., ~400 degrees Celsius). It is a fact that all HTFs will thermally degrade over time and so it is important to monitor this to ensure that an early intervention can be taken if a problem starts to appear. The objective of HTF monitoring being to keep the HTF and plant operational for as long as is possible. Routine sampling and chemical analysis is used to assess the physiochemical properties of a HTF. For this to be done effectively, it is important to understand the properties of a virgin HTF and then to assess the rate of thermal degradation over time. Carbon residue, total acid number and closed flash point temperature are routinely measured in the laboratory and the current study proposes their use to assess the extent of thermal cracking and oxidation, two common pathways through which a HTF thermally degrades. This was done for mineral and BDObased HTFs to emphasize the background as to why BDO-based HTFs are used in CSP plants. The findings of this assessment are presented herein. Future work should consider using a similar approach to assess the condition of other HTFs commonly used in industrial applications.