The potential of hydrogel copolymer membranes in clinical sensors, based on fibre optics, is addressed. The particular properties of the membranes of relevance in this application are the ease of refractive index modulation and the potential of the hydrogel to act as a permselective barrier in which a colorimetric agent may be immobilized. The results presented illustrate the complexity of colorimetric and refractive index effects together with their dependence on pH and tonicity for hydrogels of a given composition range. The incorporation of an acryloyl-functionalized reagent (bromopyrogallol red) is used to illustrate the way in which a working pH sensor based on these combined properties may be designed and fabricated.