Polymers of intrinsic microporosity (PIMs) are a unique class of soft materials that, unlike rigid porous materials such as zeolites or carbons, exhibit both vapor adsorption (nanopore filling) and absorption (polymer plasticization or swelling). While adsorption depends on pore structure and surface functionalization, absorption is governed by polymer matrix chemistry. Since both adsorption and absorption increase sorbent mass (vapor uptake), gravimetric and volumetric characterization methods exhibit severe limitations in isotherm interpretation. Thus, distinguishing between vapor adsorption and absorption remains a key challenge for understanding subnanometer-scale processes, which play a crucial role in many emerging applications of PIMs, including gas separation, water purification, organic solvent nanofiltration and electrochemical energy storage/conversion. Herein, we present an in situ ellipsometric approach that distinguishes between adsorption and absorption based on refractive index and thickness changes in thin PIM films. Four PIMs of varying polarity are studied across multiple vapors. Disentanglement is achieved using classical physisorption theory and a Flory-based polymer swelling model. This method reveals how polymer chemistry governs sorption mechanisms and offers a general framework for characterizing nanoporous soft materials.