Speleothems are key archives for palaeoclimatological study, yet current methods for palaeotemperature records are often affected by processes independent to temperature. Phosphorus is a ubiquitous component of speleothem calcite in caves, yet its efficacy as a palaeothermometer has not yet been fully explored. The fractionation of phosphate-oxygen-isotopes (δ18OPO4) to temperature by pyrophosphatase (PPase) enzymes is frequently mentioned throughout the literature as a chemical thermometer but has yet to be tested in speleothems. This dissertation therefore aimed to evaluate the efficacy of δ18OPO4-thermometry in a contemporary cave monitoring and palaeoclimatic context. This was accomplished through δ18OPO4-temperature-equilibration experiments of drip-waters and speleothem material collected from Poole’s Cavern, Buxton. Also tested were two speleothems grown over the last 100-years from Ethiopia and the UK, and two palaeo-δ18OPO4-archives from Australian speleothem YB-F1 (99-37 ka) and the Archean Ocean (3.2-3.5 Ga). Results show that a PPase fractionation equation for Poole’s Cavern of 1000퐿푛∝(푃푂4−퐻2O)=15.801∙(1000푇)−29.106 (R2 = 0.69 P