Effects of sterilization by electron beam (E-BEAM) on paclitaxel (1) mixed with poly(DL-lactide-co-glycolide) (PLG) in reservoirs of COSTAR Stents are examined by using liquid chromatography-mass spectrometry (LC-MS-MS) techniques with information-dependent acquisition (IDA). Numerous degradation products of 1 are formed in a β-radiation dose-dependent manner to give plethora of low-level degradants. This behavior, together with multiple interferences from PLG-related compounds, creates considerable challenges for analysis of the drug/PLG mixtures. IDA methods with different survey scans are proven to be very efficient in elucidating degradation pathways and in identifying numerous products. Combined LC-MS-MS results from analysis of sterilized drug substance and stents indicate that water addition and oxidative processes together with the isomerization are largely responsible for degradation of 1 under E-BEAM sterilization conditions used.