Fiber-optic microsensors with a tip diameter of similar to 140 mu m have been developed that enable simultaneous measurement of dissolved oxygen (DO) and pH, and of DO and temperature M, respectively. The tip of the optical fiber was covered with sensor compositions based on luminescent microbeads that respond to the respective parameters by a change in the decay time, intensity of their luminescence, or both. The use of microbeads enables the ratio of the signals to be easily varied, reduces the risk of fluorescence energy transfer between indicator dyes, and reduces the adverse effect of singlet oxygen that is produced in the oxygen-sensitive beads. The sensor chemistry for DO/pH was modified.