Fiber-loop ring-down spectroscopy (FLRDS) is a recently developed absorption spectroscopic technique suitable for very small liquid samples. It is based on measurements of the optical decay constant of laser intensity in a loop made of optical waveguide material. This decay constant changes as small liquid samples containing absorbing species are introduced into the loop. In this report, it is demonstrated that one can also obtain the optical decay constant using a continuous wave laser beam that is intensity modulated and then coupled into an optical fiber loop. The inherent exponential decay in the fiber loop introduces a phase shift of the light emitted from the loop with respect to the pumping beam. By measuring this phase shift, one can readily determine the concentration of the analyte introduced between the two fiber ends and a model is established to describe the relationship. It is demonstrated that this technique, dubbed phase-shift fiber-loop ring-down spectroscopy (PS-FLRDS), is well suited as an absorption detector for any flow system in which the optical absorption path is limited by the instrument architecture. By measuring the phase angle as a function of concentration of 1,1'-diethyl-4,4'-dicarbocyanine iodide in dimethyl sulfoxide, the detection limit was determined as approximately 6 microM for a 30-40-microm absorption path. A temporal resolution of approximately 100 ms was demonstrated by a rapid displacement of the solutions between the two fiber ends. Proof-of-principle use of the PS-FLRDS detection in capillary flow systems using a commercial four-way microcross established that the alignment of the fiber and the capillary can be made simple and effective, while retaining both a low detection limit and a fast response.