Poly(methyl methacrylate) (PMMA) discs in various molecular weights (M-w) were prepared by free-radical polymerization. Pyrene (Py) was introduced during polymerization as a fluorescence probe. In situ steady state fluorescence experiments were performed to monitor desorption processes in chloroform solvent. When the PMMA discs were in chloroform, desorption of Py molecules were monitored by observing the change of Py fluorescence intensity I-P. To understand the mechanism of desorption, two different experiments were carried out in the meantime. In the first experiment, the weight of swelling and dissolving PMMA discs against dissolution time was monitored. The parallel experiment was conducted by measuring fluorescence intensity I-P from the desorbing Py molecules. Then, a set of desorption experiments were performed by monitoring I-P for various discs with different molecular weight of PMMA. A Fickian diffusion model was employed to quantify the fluorescence data produced from the swelling PMMA discs to measure desorption coefficients D-d. The measured D-d values decrease as the molecular weight of PMMA is increased in the swelling glass. (c) 2006 Wiley Periodicals, Inc.