The rate and mechanism of hydrolytic degradation and erosion determine the utility and performance of bioresorbable polymers for tissue-engineering and drug-delivery applications. Nevertheless, changes in polymer morphology at the nanoscale caused by degradation and erosion, particularly those associated with the spatial distribution of water, are not well understood. We exploit recent imaging advances based on spatially resolved electron energy-loss spectroscopy (EELS) in the cryo-scanning transmission electron microscope to quantitatively map the morphological changes of a degrading and eroding random copolymer of DTE (desaminotyrosyl-tyrosine ethyl ester) and PEG (poly(ethylene glycol)). Weight-gain measurements indicate that this polymer contains 12 wt % water after 48 h of immersion in water at 20 degrees C. After immersion for 5 months, EELS shows that the average water content increases from 11.8 +/- 0.8 to 14.9 +/- 1.0 wt %, but 15 nm resolution compositional maps do not reveal any morphological changes. After immersion for 12 months, however, compositional mapping resolves a network of PEG-depleted regions with characteristic sizes of 50-100 nm. These PEG-depleted regions are enriched in water. They correspond to high-diffusivity paths through which PEG-rich fragments eroded. Exposure to water at 37 degrees C for 2 additional weeks leads to an even more developed network of channels about 100 nm in size. These findings suggest that the development of high-diffusivity pathways at nanolength scales plays a key role in the early stages of bioresorption in bulk-eroding polymers, and they demonstrate that electron energy-loss spectroscopy in the STEM is an effective new tool to quantitatively study the morphological changes associated with polymer hydration, degradation, and erosion at these fine length scales.