In our lab, we use the liquid linear alkylbenzene (LAB) cocktail mixed with 2g/L 2,5 diphe-nyloxazole (PPO) as the organic scintillator for radiation detection. The new LAB cocktails are stored in the transparent glass bottles provided by the company. Although they are exposed to the daylight lamp in the lab, they are always very clear(see picture 1, the sample was put in a small glass vial). However, when we put some liquids into a transfer system, they turned to be very yellow after about two weeks (see picture 2). In the system, the LAB liquid was first stored in a large Acrylic vessel (acrylic is proved to be compatible with the LAB) and then went through the stainless steel tubes and valves (from Swagelok company), finally went into a small acrylic container or a glass jar for preventing spilling. There is no heating during the experiment, just some pressure changes created by a pump to make the LAB transfer from the large vessel to the small container. The LAB liquid may also contact with the rubber from O-ring for vacuum sealing, the Teflon tape, the plastic(PTFE) gas tube, copper lid, etc. So in our case, what may cause the yellowing for the LAB?