Background Whilst it is assumed that water plays a crucial role in homeopathy, it is unclear what specific functions water has in the generation, propagation and maintenance of homeopathic potencies. Solvatochromic dyes have demonstrated themselves to be reliable indicators of the presence of potencies and so are well placed to begin to address these questions. Objective The aim of the current study was to ascertain whether pure water could be potentised. If pure water can be potentised, this would indicate that water plays a crucial role in the generation of homeopathic potencies. If it cannot be potentised, then this result points to water being an inert carrier and medium for the potentisation of substances placed in it. Methods A specially designed potentiser capable of consistently and reliably producing homeopathic potencies to rigorous standards (Pinkus Potentiser) was used to run up potencies of “Water 10M” and Arsenicum 10M in two interchangeable stainless-steel vials. “Water 10M” and Arsenicum 10M, along with unsuccussed control water, were then tested against solutions of the solvatochromic dye phenol blue by following the absorbance of the dye at 658 nm. Results “Water 10M” gave results no different statistically from unsuccussed water (p > 0.05) with no change over the time course of the experiments, whereas Arsenicum 10M gave statistically different results (p < 0.001) with increasing divergence from both “Water 10M” and unsuccussed water over time. Similar results were obtained when the vials in which Arsenicum 10M and “Water 10M” were produced were exchanged, so that the vial in which Arsenicum 10M had been produced was then used to produce “Water 10M” and vice versa, indicating the results could not be explained by any anomalies in the production vials themselves. Conclusion Results using the solvatochromic dye phenol blue to detect and measure homeopathic potencies indicate that bulk water itself does not potentise and that unsuccussed water and water that has been succussed are no different. This indicates that what is potentised during cycles of dilution and succussion are the substances placed in water and not the water itself. In turn, this indicates that water is simply a carrier for the homeopathic signal. Evidence from the practice of homeopathy supports this conclusion.