Cytochrome P450 was first shown to be a hemoprotem in 1964 (1). It has as its prosthetic group a noncovalently bound iron protoporphyrin IX, a form of heme found in b-type cytochromes, like cytochrome b, hemoglobin, and myoglobm. This type of heme prosthetic group can be removed from the protein by acidic acetone or by alkali and pyridine. There are a number of methods that can be used for spectrophotometrrc analysis of hemoproteins, each useful for a different purpose. In this chapter, we will provide methods for quantifying cytochrome P450, as well as methods for studying substrate interaction with the purified protein, the hemoprotem in membranes, and in tissue homogenates. The ability of those hemoproteins with an available 6^th coordination position to bind small molecules such as cyanide, carbon monoxide, ethyl isocyanide, etc., has provided means for their analysis and quantification. In the case of cytochrome P450, the binding of carbon monoxide permitted demonstration of its mvolvement in the monooxygenase reaction.