This study provides a comprehensive evaluation of the degradation and stability of crosslinked polyethylene (PEX) pipes designed for high temperature borehole thermal energy storage (BTES) systems. Special at tention is given to a novel type of PEX pipe manufactured using a photo initiated crosslinking (PEX-e) process. Two formulations, PEX-e1 and PEX-e2, were assessed and compared with commercial peroxide-crosslinked polyethy lene (PEX-a) and non-crosslinked bimodal polyethylene (PE100) pipes. The first phase of the study focused on the characterization of the pipes in their as-received condition to evaluate their morphology and properties post processing. The degree of crosslinking was quantified through gel content and the crosslink density was determined through dynamic mechanical analysis (DMA). Phase composition and molecular mobility were examined using sta tic 1H nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC), while oxidative stability was evaluated using oxidation induction time (OIT) measurements. Both PEX-e formulations exhibited elevated crosslin king degrees and high OIT values. The second phase of the study involved a detailed analysis of thermo-oxidative degradation and stability through ageing tests. This was achieved by immersing pipes in water at BTES service tem peratures for 210 days. Changes were monitored using advanced analytical techniques, including fourier transform infrared spectroscopy (FTIR), 1H and 13C NMR, colorimetry, and DMA. Results revealed that physical ageing do minated the first 30 days, while over time, chain scission was identified as the primary degradation mechanism, with PEX-a being the most severely affected. Interestingly, despite the high OIT levels, both PEX-e exhibit accelerated AO depletion during prolonged exposure to hot water. These findings highlight the importance of long-term ageing tests at service conditions and underscore the need for further optimization of PEX-e formulations specifically tailored for BTES service conditions.