Significance Although the theoretical model of Monod, Wyman, and Changeux (MWC) is one of the most influential and highly cited theoretical models in bioscience, it fails to explain allosteric effects in hemoglobin, the paradigm of allostery, because their model considers only quaternary preequilibria. By using a new kind of laser photolysis experiment, measurements of ligand rebinding kinetics for transient hemoglobin conformations trapped by encapsulation in silica gels support the simplest possible extension of the MWC allosteric model to include tertiary in addition to quaternary conformational preequilibria. While the MWC model provides a qualitative explanation for allostery in many multisubunit proteins, quantitative analysis will most probably require the extension used here to explain our new results.