New ethylene vinyl alcohol copolymer EVOH nanocomposites based on zeolite particles were processed by solution or melt-mixing to synthesise potential encapsulants for flexible organic solar cells (OSC). The photochemical behaviour of pristine EVOH copolymer and the influence of zeolite particles with different size and increasing nanoparticle loading were studied to evaluate the photostability of such packaging materials. Photooxidation of pristine EVOH films (with two different ethylene unit contents (27% or 44%) was performed under accelerated artificial conditions that allow for the proposal of a comprehensive mechanism of EVOH oxidation. The photochemical behaviour of EVOH/zeolite nanocomposites with different morphology and size of particles was compared, revealing a higher oxidation of the polymer in the presence of zeolite particles, which can be mainly attributed to iron impurities.