A novel method for applying cavity ring-down spectroscopy in the liquid phase, compatible with LC analyses, is presented. The core of the setup is a home-built cavity ring-down flow cell (cell volume 12 μL) that is constructed using a silicon rubber spacer, which is clamped leak-tight between two high-reflectivity mirrors. The mirrors are in direct contact with the liquid flow, which provides for a small path length and short ring-down times. Inside the cavity there are no windows, reflection losses, or Brewster angles to be considered. Due to the small size of the presented cavity geometry, the setup can be implemented in conventional-size LC apparatuses. With a flow injection setup, a detection limit of 2.5 nM was obtained for Crystal Violet in ethanol, and the linear dynamic range of the system is at least 2 orders of magnitude. The method has the potential to become a powerful alternative for commercial LC UV/visible absorbance detectors.