Sorption often shows ‘irreversible' behaviors such as hysteresis, enhanced repeat sorption (the “conditioning effect”), and partial immobilization. Elucidation of their causes is critical. A glassy polymer model for sorption has recently evolved. The glassy (stiff-chain) state is characterized by inefficient packing of macromolecules, leaving it with persistent excess free volume. The solute sees it as internally microporous. The pores (“holes”) are not fixed, but may expand/contact under the influence of temperature or sorbate. Irreversible pore deformation as molecules enter/leave holes or proto-holes is believed the root cause of hysteresis in polymers. We document irreversible sorption of hydrophobic compounds in SOM (humic acid and high OC soils) and provide evidence for pore deformation. 14-C isotope exchange (IE) experiments were performed at equilibrated bulk sorption and desorption points at constant bulk chemical concentration. The finding of complete IE in systems demonstrating bulk hysteresis rules out common artificial causes. Comparison of normalized uptake and release kinetics of label versus bulk solute at different concentrations provided further evidence for a pore deformation cause. A small fraction of initial sorbed mass became highly desorption-resistant, possibly by a ‘matrix collapse' effect. Further studies examined the conditioning effect and its relaxation by low-temperature annealing. The conditioning effect was observed for two natural solids and a glassy polymer, but not for a rubbery polymer. Conditioning effect for the natural solids (sorption enhancement at 22 oC) relaxed upon sample annealing, similarly to free volume and enthalpy of glassy polymers. Relaxation increased with annealing temperature (45 - 91 oC) and followed a double exponential rate law with a non-zero constant that varied inversely with temperature. At environmentally relevant temperatures the so-conditioned solid may “never” completely relax. The combined results of IE and conditioning-annealing studies provide compelling evidence for the glassiness of SOM and irreversible structural expansion as causative of irreversibility.