CHARMM force field parameters have been developed to model nonprotein bound five coordinate ferriheme (ferriprotoporphyrin IX) species in aqueous solution. Structures and solvation were determined from molecular dynamics (MD) simulations at 298 K of monomeric [HO-ferriheme](2-), [H2O-ferriheme](-), and [H2O-ferriheme](0); pi-pi dimeric [(HO-ferriheme)(2)](4-), [(H2O-ferriheme)(HO-ferriheme)](3-), [(H2O-ferriheme)(2)](2-), and [(H2O-ferriheme)(2)](0), and mu-oxo dimeric [mu-(ferriheme)20]. Solvation of monomeric species predominated around the axial ligand, meso-hydrogen atoms of the porphyrin ring (H-meso), and the unligated face. Existence of pi-pi ferriheme dimers in aqueous solution was supported by MD calculations where such dimers remained associated over the course of the simulation. Porphyrin rings were essentially coplanar. In these dimers major and minor solvation was observed around the axial ligand and H-meso positions, respectively. In mu-oxo ferriheme, strong solvation of the unligated face and bridging oxide ligand was observed. The solution structure of the mu-oxo dimer was investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS spectrum obtained from frozen solution was markedly different from that recorded on dried mu-oxo ferriheme solid. Inclusion of five solvent molecules obtained from spatial distribution functions in the structure generated from MD simulation was required to produce acceptable fits to the EXAFS spectra of the dimer in solution, while the solid was suitably fitted using the crystal structure of mu-oxo ferriheme dimethyl ester which included no solvent molecules.