Ferumoxides, dextran-coated superparamagnetic iron oxide (SPIO) particles, form ferumoxide-transfection agent (FE-TA) complexes that are internalized into endosomes/lysosomes and have been used to label cells for in vivo MRI tracking and localization studies. A better understanding of the physical state of the FE-TA complexes during endocytosis could improve their use. The purpose of this study was to measure the rate of the degradation of iron particles under varying physiological conditions. FE-TA complexes were incubated in seven different buffers containing different chelates with different pH. Reducible iron concentrations, T2 relaxation rates and gradient echo (GRE) magnetic resonance images (MRI) were obtained from each condition immediately after incubation and at 6, 24, 48, 72 and 96 h and days 7, 14 and 21. The dynamics of FE-TA in the endosome/lysosomes within the cells were visualized with electron microscopy. Sodium citrate buffer at pH 4.5 rapidly dissolved FE-TA complexes. However, FE-TA complexes were less soluble in the same buffer at pH 5.5. Similarly, FE-TA complexes were not readily soluble in any of the other buffers with or without chelates, regardless of pH. Electron microscopic images showed degraded FE-TA in some intracellular endosome/lysosomes between days 3 and 5. In the cellular environment, some of the FE-TA-containing endosomes were found to fuse with lysosomes, causing rapid dissociation at low pH and exposing the iron core to chelates that resulted in soluble Fe(III) within the lysosomes. The studies presented represent a first step in identifying the important cellular environmental parameters affecting the integrity of FE-TA complexes.