Abstract: An optical fiber is used in conjunction with a sensor capable of sensing more than one analyte. A first fiber optic sensor cell is used for measuring any combination of ionic species and a second fiber optic sensor cell is used for measuring all gaseous species. In one embodiment, a doped polymer is formed utilizing a hydrophilic polymer which immobilizes a pH sensitive dye and a potassium sensitive fluorescence dye, allowing pH to be measured by detecting the color change of the pH sensitive dye, and the potassium ion concentration to be measured by the change in fluorescence intensity of the potassium sensitive dye. In an alternative embodiment, a doped polymer is formed utilizing a hydrophilic polymer which immobilizes a calcium or sodium ion sensitive fluorescence dye in order to form either a combined pH/calcium sensor or a combined pH/sodium sensor, respectively.