The intermembrane space is the region between the inner membrane and the outer membrane of a mitochondrion or a chloroplast. The main function of the intermembrane space is nucleotide phosphorylation.
Channel proteins called porins in the outer membrane allow free movement of ions and small molecules into the intermembrane space. This means that it is essentially continuous with the cytosol in terms of the solutes relevant for the functioning of these organelles. Enzymes destined for the mitochondrial matrix or the stroma can pass through the intermembrane space via transport through translocators. These are known as translocase of the outer mitochondria membrane (TOM) and translocase of the inner mitochondrial membrane (TIM) in mitochondria and translocase of the outer chloroplast membrane (TOC) and translocase of the inner chloroplast membrane (TIC) in chloroplasts. It tends to have a low pH because of the proton gradient which results when protons are pumped from the mitochondrial matrix into the intermembrane space during electron transport. The structures responsible for this are coenzyme Q, NADH coenzyme Q oxidoreductase complex (complex I), succinate-coenzyme Q oxidoreductase complex (complex II), and coenzyme Q-cytochrome c oxidoreductase complex (complex III).
The intermembrane space of the chloroplast is extremely small, somewhere from 10-20 nm thick.