The surface properties of the active ingredients in AMI colloidal, superparamagnetic iron oxide magnetic resonance (MR) contrast agents are described. Scanning electron microscopy/energy dispersive X-ray elemental analyses and diffuse reflectance Fourier transform infrared spectroscopy (FTIR) spectra of ferumoxsil (AMI-121 drug substance) were consistent with the presence of a monolayer of H2NCH2CH2NHCH2CH2CH2Si(O-)3 siloxane monomer or dimer. The X-ray photoelectron spectra (XPS) of ferumoxsil are also consistent with complete coverage of the iron oxide surface with a monolayer of siloxane. The static secondary ion mass spectra (SSIMS) of ferumoxsil showed that the siloxane film is covalently bonded (i.e., Si-O-Fe bonds) to the iron oxide surface. The FTIR of ferumoxides (AMI-25) and Ferumoxtran (AMI-227) showed only adsorbed dextran. The XPS spectra of the dextran-coated colloids showed that Ferumoxtran has a thicker layer of dextran than ferumoxides iron oxide particles (approximately 5 and approximately 3 nm, respectively). The SSIMS spectra of these dextran-coated colloids showed only low mass fragments due to the adsorbed dextran. The nature of the interactions of the dextran coating with the iron oxide surfaces of ferumoxides and Ferumoxtran is discussed.