Understanding the degradation behavior of polymeric microspheres is crucial for the successful application of such devices in controlled drug delivery. The degradation mechanism of poly(lactic-co-glycolic acid) (PLGA) microspheres inside phagocytic cells is not known, but different models for degradation in aqueous solution have been proposed. We have used confocal Raman spectroscopy and imaging to study the intracellular degradation of PLGA microspheres inside individual macrophages. Our results show that ingested microspheres degrade in a heterogeneous manner, with a more rapid degradation in the center. Comparison of Raman spectra from degrading beads with those of uningested beads reveals that ester hydrolysis occurs throughout the phagocytosed microspheres, with a selective loss of glycolic acid units. Furthermore, we show that PLGA degradation is a cell-mediated process, possibly caused by the low pH of the phagosome and/or the presence of hydrolytic enzymes. In conclusion, we have demonstrated that the chemical composition of degrading polymers inside cells can be probed by Raman spectral imaging. This technique will expand the capabilities of investigating biomaterial degradation in vivo.