Confocal laser scanning microscopy (CLSM) was utilized to explore noninvasively the diffusion mechanisms of water-soluble, low molecular weight fluorescent probes in photopolymerized poly(ethylene glycol) (PEG) hydrogels. First, the simple case of 1-D diffusion in uniformly loaded PEG disks was characterized. Dye distributions in PEG disks were imaged using CLSM during release experiments and quantitatively evaluated through image analysis for comparison to Fickian diffusion models. Dye diffusion coefficients were determined by fitting Fickian diffusion equations to experimentally measured concentration profiles. Effective diffusion coefficients were on the order of 10(-8) cm(2)/s, comparing favorably to literature values. Second, the time-dependent distributions of two separate probes were monitored simultaneously to explore the multicomponent tracking capabilities of CLSM in PEG hydrogels. Finally, a more complex dye loading system was examined by embedding microparticles loaded with fluorescent dye in PEG matrices and characterizing 3-D radial dye diffusion.