Polyunsaturated fatty acids (FAs) of intravenous (IV) lipid emulsions can peroxidize to potentially harmful lipid hydroperoxides. In order to assess in vitro peroxidation of IV fat emulsions in all-in-one (AIO) admixture bags, an iodometric titration to determine lipid hydroperoxide content expressed by the peroxide value (PV) and a gas-liquid chromatographic (GLC) assay to determine changes of the FA pattern were established. A long-chain triglyceride (LCT) and medium-chain triglyceride-LCT emulsion were compared for the PV and the pH during storage at room temperature and daylight in AIO bags made of ethylvinylacetate (EVA) and polypropylene:polyamide 7:3 (V90). In contrast to storage in glass bottles, significant peroxidation was detected in both emulsions with 0.5-3.4 mmol peroxides/L after 28 d (150 times the control PV). A pH drop of at least 0.3 (EVA) and 1.2 (V90) units was measured. Initial PVs and peroxidation kinetics of the emulsions were different; V90 material showed better barrier properties against oxygen. PV was increased by higher temperature and light exposure. The FA pattern of an LCT emulsion with a PV > 6 (storage: 40 degrees C in a dark room for 28 d in AIO bags) assayed by GLC remained unchanged. The iodometric peroxide and the GLC assay were reproducible and easy to handle. Only the iodometric method was sensitive enough to detect peroxidation effects (detection limit: 0.02 mmol peroxides/L). IV fat emulsions can be checked for lipid hydroperoxide content with the rapid iodometric assay to guarantee optimal quality of IV lipids used for AIO admixtures. To prevent peroxidation, lipids in AIO bags should be stored light-protected in a refrigerator an oxygen-tight overwrap is mandatory for extended periods.