In this chapter, we describe a method to prepare photosystem I (PSI) submembrane fractions derived from the chloroplast stroma lamellae of spinach chloroplasts. These preparations retain the cytochrome b6/f complex and a pool of about 11 plastoquinones per P700. The PSI submembrane fractions are thus able to perform both cyclic and linear electron transport reactions from various artificial electron donors to oxygen or methylviologen. They are useful to study both PSI and cytochrome b6/f complex activities in a nearly native form without interference from photosystem II.