ATP synthesis occurs on the

ATP synthesis occurs on the inner membrane of mitochondrion.
The inner mitochondrial membrane contains many copies of a protein called the F0F1ATPase. This is also called ATP synthase. It consists of two parts: the F0 component spans the membrane and provides a channel for protons to move into the matrix from the intermembrane space.
The ATP synthase complex is located in the inner membrane of mitochondria, with ATP synthesis reaction occurring on the membrane side toward the matrix compartment.