The GTP-dependent interaction of ADP ribosylation factor 1 (ARF1) with Golgi membranes is required for the binding of cytosolic coatomer proteins to those membranes. Whereas both GTP and GTP gamma S can support coatomer binding to membranes, by using partially purified components, GTP-driven binding is rapidly reversible (t1/2 of 2 min) while that driven by GTP gamma S is more stable (t1/2 of over 30 min). In the presence of GTP, aluminum fluoride, an activator of trimeric G proteins, promotes the stable ARF-dependent binding of coatomer to membranes, even though this reagent does not itself activate ARF. Aluminum fluoride appears to act, like GTP gamma S, to make the binding of coatomer relatively irreversible. It acts to inhibit ARF-GTP hydrolysis catalyzed by the membrane and thus makes the ARF-GTP active state persistent. This effect is not dependent on the presence of any cytosolic component, such as the coatomer. The number of molecules of ARF that can be protected from hydrolysis by aluminum fluoride, however, is only a fraction of the total amount of ARF that can bind to membranes in the presence of GTP gamma S. We propose that this population defines a set of binding sites that are sufficient for coat protein assembly onto the membrane.