AbstractWe studied the effects of some organic cosolvents (monohydric alcohols and amides) on the reaction of hemoglobin with oxygen. We present evidence showing that our data can be analyzed within the framework of the Monod‐Wyman‐Changeux model and that the main effect of cosolvents is to alter the T ⇄ R conformational equilibrium of hemoglobin, without significantly affecting the intrinsic oxygen dissociation constants. Following a previously described phenomenological approach, the overall effects have been separated into effects related to the variation of the bulk dielectric constant of the solvent and effects not related to the variation of this con