Abstract: It has been shown in a previous paper that starch formation in leaf discs floating on sucrose solution is greatly enhanced by illumination. By using various inhibitors it has been found the increase is probably due to increased phosphorylation. The present paper reports further experiments with arsenate. It was found that in maleic buffer, arsenate inhibits starch formation in light 50% at 5 × 10-4 M and 100% at 2× 10-3 M. In phosphate buffer (1/30 M) it inhibits only 20% and 50% at the respective concentrations . The arsenate inhibition and its competitive nature with phosphate confirmed the previous conclusion that starch formation is through phosphorylation. Further experiments were made in air, in oxygen and in nitrogen. In dark, starch formation is proportionally to oxygen tension, but the maximum amount of starch formed even in flowing oxygen is far less than that in light without oxygen . It is clear therefore that oxidative phosphorylation plays only a minor role in comparas on with the phosphorylation due to light (Photosynthetic phosphorylation of Arnon). In light, starch formation shows no correlation with oxygen partial pressure. However, a very small amount of oxygen appears to be necessary for optimal activity, possibly connected the formation of co-factors. Microscopic examination of leaf sections showed that the starch formed is localized in the chloroplasts, the result collaborates the contention of photosynthetic phosphorylation.