A lecture demonstration illustrating changes in the visible spectra of oxygen transport proteins upon reversihle oxygen binding is reported. The coloration upon exposure to oxygen is dramatic. Deoxyhemoglobin changes from purple to red, deoxyhemocyanin is transformed from colorless to blue, while colorless deoxyhemerythrin turns red. This demonstration illustrates an interesting example of a chemically reversible reaction and is used in the discussion of basic thermodynamic properties. A comparison of the physical characteristics of these oxveen storaee and transn.o rt ~ro- A teins is provided to illustrate the stru&ral diversity associated with these proteins having a similar biological role.