Properties of polymeric microgels are influenced by the internal polymer cross-linked structure, but tools to quantitatively analyze this internal structure are limited. With the finding that polymer networks alter the diffusivity and subsequent excimer formation of pyrene, this study used the ratio between pyrene excimer and monomer emission to determine the number of cross-links (N) and average pore size (xi) in poly(ethylene glycol) diacrylate (PEGDA) microgels. A calibration curve to relate pyrene emission to N and xi in PEGDA hydrogels was prepared and used to calculate N and xi in PEGDA microgels. The pyrene emission indicated that PEGDA microgels had a higher cross-linking density and a smaller average pore size when compared with bulk cross-linked hydrogels of the same PEGDA concentration. The analytical method demonstrated in this study may be useful for fine-tuning polymeric microgel properties for a broad array of applications. (C) 2009 Elsevier Ltd. All rights reserved.