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An annular ultraviolet (UV) reactor as a single vertical segment and 4 segments of variable length connected in series were used to study inactivation kinetics of Escherichia coli and UV-dose distribution. Both configurations produced linear inactivation curves in model juices. The rate of inactivation decreased as the solution absorption coefficient increased. UV doses for 90% reduction were determined and varied from 4.5 to 6.5 mJ/cm2 in a vertical setup. To study the UV-dose distribution within a reactor, E. coli was injected and yielded the range from 5.6 to 11.1 mJ/cm2. A modified biodosimetry method using the injection of bacteria as a tracer can be used to evaluate dose delivery in UV reactors.