The sensing element is a bead of commercially available anion exchange resin formed by introducing a quaternary ammonium group onto porous crosslinked polystyrene. The diameter of the bead is 20-40% larger in water than in organic solvents investigated in this study including acetone, ethanol, methyl ethyl ketone, 3-pentanone, 4-methyl-2-pentanone and 3-heptanone. The size of the bead varies continuously with the activity of water. In the sensor bead size changes are coupled to movement of a reflecting diaphragm. Movement of the diaphragm changes the intensity of light reflected into an optical fiber. Light intensity is measured as a function of water activity. The results demonstrate that it is possible to sense reversibly and continuously the activity of water in an organic solvent using the phenomenon of the polymer swelling. Problems encountered with cracking of the polymer bead with use and swelling of the diaphragm in the sensor can be addressed by modifying the polymer formulation and changing the sensor design.