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A new fiber-optic sensor for simultaneous measurement of water-soluble analytes and temperature with polymer-coated fiber Bragg gratings (FBGs) is proposed. As an application of the approach, simultaneous monitoring of the concentration of sugar or potassium chloride (KCl) and temperature has been achieved. Changes in these environmental parameters result in different extents of either red- or blue-shifts of the Bragg resonance wavelengths of the gratings. It has been found that polyimide-coated FBG responds to variations of both temperature and concentrations of soluble analytes, while acrylate-coated FBG is sensitive to environmental temperature only. The experimental results showed that the temperature sensitivity of the acrylate-coated FBG, temperature, sugar, and KCl concentration sensitivities of the polyimide-coated FBG are 0.0102 nm/degC , 0.0094 nm/degC, 0.0012 nm/degBx , and 0.0126 nm/M, respectively. The sensing mechanism of the polyimide-coated FBG lies in the hygroscopic properties of the polyimide coating, which result in the change of the strain of the fiber and, thus, the optical properties of the grating. Since the sensor detects the analytes that swell the polyimide coating and different analytes induce different swelling effects, the sensor can detect different analytes without prior knowledge once a calibration curve is developed.