A concept of a microfluidic fluorescent chemical sensing system is presented and demonstrated as a sensor for measurement of dissolved oxygen in water. The system utilizes on-line mixing of a fluorescent reagent with the analyzed sample, while it measures the fluorescence decay time of the mixture. The system is built entirely out of silica capillaries and optical fibers, and allows for very low consumption of the reagent (of the order of mL/month) and the analyzed sample (of the order of L/month). The proposed system can, thus, be applied to continuous on-line measurements, while utilizing a broad variety of different and proven fluorescent reagents or dyes. The proposed system allows for the use of relatively high-excitation light powers, as the flow-through concept of the system reduces the probability of the appearance of bleaching, heating, or other unwanted effects on the fluorescent dye/reagent caused significantly by the excitation light. The high amplitudes of fluorescent optical signals captured by an optical fiber allow for low-noise and high-bandwidth optical signal detection, and, consequently, the possibility for utilization of reagents with nanosecond fluorescent lifetimes.