Biological membrane fusion employs divalent cations as protein cofactors or as signaling ligands. For example, Mg2+ is a cofactor for the N-ethylmaleimide-sensitive factor (NSF) ATPase, and the Ca2+ signal from neuronal membrane depolarization is required for synaptotagmin activation. Divalent cations also regulate liposome fusion, but the role of such ion interactions with lipid bilayers in Rab- and soluble NSF attachment protein receptor (SNARE)-dependent biological membrane fusion is less clear. Yeast vacuole fusion requires Mg2+ for Sec18p ATPase activity, and vacuole docking triggers an efflux of luminal Ca2+. We now report distinct reaction conditions where divalent or monovalent ions interchangeably regulate Rab- and SNARE-dependent vacuole fusion. In reactions with 5 mm Mg2+, other free divalent ions are not needed. Reactions containing low Mg2+ concentrations are strongly inhibited by the rapid Ca2+ chelator BAPTA. However, addition of the soluble SNARE Vam7p relieves BAPTA inhibition as effectively as Ca2+ or Mg2+, suggesting that Ca2+ does not perform a unique signaling function. When the need for Mg2+, ATP, and Sec18p for fusion is bypassed through the addition of Vam7p, vacuole fusion does not require any appreciable free divalent cations and can even be stimulated by their chelators. The similarity of these findings to those with liposomes, and the higher ion specificity of the regulation of proteins, suggests a working model in which ion interactions with bilayer lipids permit Rab- and SNARE-dependent membrane fusion.