X-ray absorption spectroscopy has been used to study the dimeric iron center in azidomethemerythrin from Phascolopsis gouldii. Absorption edge data confirm that the two iron atoms are present as Fe(III) and suggest a hexa-coordination site for each of the iron atoms. The extended x-ray absorption fine structure (EXAFS) analysis provides direct structural evidence of a mu-oxo bridge between the two iron atoms at an average Fe-O distance of 1.71-1.76 A. Analysis using a multiple-scattering formalism calculates upper limits of 165 degrees for the Fe-O-Fe bridging angle and 3.38 A for the Fe-Fe distance. This result agrees with current crystallographic models being determined by refinement of structures of two azidomethemerythrins.