The behavior of ferrihemoglobin and ferrimyoglobin in widely varying concentrations of the lowest four alcohols has been studied by optical and electron paramagnetic resonance absorption spectroscopy. Methanol and ethanol, at concentrations too low to cause general conformational destabilization of the protein, produce both optical and electron paramagnetic resonance absorption spectral changes in ferrihemoglobin. These changes arise from equilibrium associations, characterized by dissociation constants at 25 degrees C of about 40 and 200 mM, respectively, for the methanol-ferrihemoglobin and ethanol-ferrihemoglobin complexes so formed. Other optical spectral changes appear when the methanol concentration exceeds 3.5 M and the ethanol, 1.0 M. At concentrations lower than 0.5 M, 1- and 2-propanol produce spectral changes of this second kind. At room temperature no optical evidence has been found that the propanols associate with ferrihemoglobin in the manner of methanol and ethanol. Methanol and ethanol at low concentration have specific effects, characterized by electron paramagnetic resonance spectral differences, upon ferric alphaSH chains. All four alcohols, over a wide range of concentrations, reduce the symmetry of electron paramagnetic resonance spectra from frozen solutions of ferrihemoglobin; even at the high end of this concentration range, none of the alcohols reduces the symmetry of electron paramagnetic resonance spectra from frozen ferrimyoglobin. Ferrimyoglobin and catalase association with methanol is measurable optically; the binding is about five and sixty times weaker, respectively, for these two proteins as compared with ferrihemoglobin.