  We present in situ measurements of the relative attenuation length of the
gadolinium loaded liquid scintillator in the RENO (Reactor Experiment Neutrino
Oscillation) detectors using radioactive source calibration data. We observed a
steady decrease in the attenuation length of the Gd-LS in the RENO detectors by
50% in about four years since the commissioning of the detectors.
