Accurately assessing the interfacial composition and reactivity of (bio)polymers under controlled but environmentally relevant conditions remains challenging. This study explores the evolution of surface functionalities of novel polyesters Poly(Butylene Xylose) (PBX) and Poly(Alkyl Xylose Amide) (PAXA) under artificial degradation conditions. Employing a Knudsen Flow Reactor (KFR) and a gas-titration approach, we systematically analyze the chemical transformations occurring at the polymer’s solid-gas interface. Both polymers are derived from the same functionalized lignocellulosic sugar building block, Diglyoxylic Acid Xylose (DGAX). Virgin (V), long-term UV (UV), and short-term Ozone (O3) exposures induce specific but differing alterations in molecular integrity and surface reactivity, resulting in notable shifts in material properties and polymer structure. Contrary to the assumption that degradation increases specific surface area (SSA) and reactivity, our results reveal cases where the SSA decreases, with reactivity either increasing or decreasing based on the reactive groups available at the interface. Both polymers exhibited increased water affinity, acidification, and ozone reactivity following UV exposure. Interfacial reactivity, assessed with trifluoroacetic acid, hydroxylamine, and nitrogen dioxide, increased for PBXUV but decreased for PAXAUV. Surface hydroxyl groups in PAXA reduced 5-fold under short-term ozone and long-term UV exposure, while bulk transport kinetics of hydroxylamine altered with long-term degradation, though ozone exposure left transport mechanisms unaffected. This combined interfacial and bulk analysis approach advances our understanding of (bio)plastic degradation. It examines the role of degraded polymers as potentially more or less reactive vectors for chemicals and organisms upon environmental release while also demonstrating that polymer degradation initiates significantly earlier than previously assumed.