The crystal structure of the enzyme-product complex formed between cytochrome P450(cam) and 5-exo-hydroxycamphor has been refined to 2.2 Angstrom and compared to the enzyme-substrate complex. The product occupies the same position as the substrate with the exception that the product 5-hydroxyl group forms a weak interaction with the heme iron atom as evidenced by continuous electron density between the product OH group and the iron atom. This interaction holds the heme iron in the low-spin configuration which prevents reduction of the heme iron by the physiological electron donor, putidaredoxin. This also prevents the wasteful transfer of reducing equivalents to the product complex.