Haldane's laws, which apply to situations in which CO and O 2 compete for the four heme sites of hemoglobin, are discussed in general statistical mechanical terms. Necessary and sufficient conditions for the validity of Haldane's laws are presented and discussed for general situations in which two ligands compete for interacting active sites on a molecule. Our considerations therefore apply to a competing substrate-inhibitor-allosteric enzyme system for example, as well as to hemoglobin. A generalization of the Monod-Wyman-Changeux model for competing ligands is briefly discussed.