Oxo-Liquid Organic Hydrogen Carrier (oxo-LOHC) systems have come to prominence because of their high hydrogen storage capacity (≥ 6.3 wt%) and favourable thermodynamic properties (i.e. low dehydrogenation temperature). Given the potential scale of use of oxo-LOHCs as energy carriers, the environmental impact of these systems needs to be carefully considered. We investigated the biodegradability of twelve oxo-LOHC compounds involved in three hydrogen carrier systems. The ultimate biodegradation test demonstrated that the acetophenone-based LOHC system (H0-ACP/H8-ACP) is readily biodegradable as both compounds exhibited mineralisation above 60 % within a 10-day window (from 10 % to 60 %). In contrast, 14H-4-MBP showed potential for persistence, exhibiting less than 20 % biodegradation in both ultimate and primary biodegradation tests. H0-2-MBP exhibited low levels of primary degradation (<50 %) at both low and high initial concentrations The other oxo-LOHC compounds tested were found to be inherently biodegradable, with an average primary biodegradation rate of over 50 %, at least at one test concentration. Our findings suggest that hydrogenation of the phenyl ring and methyl substitution on the ring may decrease biodegradability. The analysis of transformation products (TPs) showed that oxo-LOHC compounds initially undergo (de)hydrogenation and/or hydroxylation, with the latter reaction potentially enhancing the estrogenic and antiestrogenic activities. Based on the 43 TPs identified, we propose biodegradation pathways for four compounds. The use of BIOWIN models generally offers a reliable estimation for the biodegradability of oxo-LOHC compounds. This study improves our understanding of biodegradation and provides guidance on the selection of inherently benign chemicals for use in LOHC systems and beyond.