In this study, we investigated the inhibitory effects of microplastics (MPs) on phenanthrene (PHE) degradation during homogeneous advanced oxidation processes (AOPs), including Fenton, ozonation, and UV/H₂O₂ processes. In the absence of MPs, PHE was completely removed in all three AOPs. However, the presence of MPs reduced the PHE removal, dependent on the amount of PHE adsorbed on MPs. An increase in MPs loading heightened the inhibitory effect on PHE removal due to enhanced adsorption of PHE onto the surface of MPs; the oxidative removal of PHE during AOPs decreased linearly with the fraction of PHE adsorbed onto MPs. These inhibitory effects, caused by PHE adsorption onto PE-MPs, were largely independent of the water matrix. Kinetic modeling revealed the second-order rate constant for the reaction of PHE adsorbed onto polyethylene(PE)-MPs with ^•OH (3.5 × 10⁸ M^-1s^-1) to be more than an order of magnitude lower than that for PHE in bulk solution (9.9 × 10⁹ M^-1s^-1). Density functional theory calculations indicated that this inhibitory effect arises from the increased activation energy required for the reaction between MPs-adsorbed PHE and ^•OH. The chemical potential of the transition state for the reaction of PHE with ^•OH on the PE-MPs surface was estimated to be 39 % higher than that for the reaction in solution.