Assembly of cognate SNARE proteins into SNARE complexes is required for many intracellular membrane fusion reactions. However, the mechanisms that govern SNARE complex assembly and disassembly during fusion are not well understood. We have devised a new in vitro crosslinking assay to monitor SNARE complex assembly during fusion of ER-derived vesicles with Golgi acceptor membranes. In Saccharomyces cerevisiae, anterograde ER/Golgi transport requires four SNARE proteins - Sec22p, Bos1p, Bet1p, and Sed5p. After tethering of ER-derived vesicles to Golgi acceptor membranes, SNARE proteins are thought to assemble into a four-helix coiled-coil bundle analogous to the structurally characterized neuronal and endosomal SNARE complexes. Molecular modeling was used to generate a structure of the four-helix ER-Golgi SNARE complex. Based on this structure, cysteine residues were introduced into adjacent SNARE proteins such that disulfide bonds would form if assembled into a SNARE complex. Our initial studies focused on disulfide bond formation between the SNARE motifs of Bet1p and Sec22p. Expression of SNARE cysteine-derivatives in the same strain produced a crosslinked heterodimer of Bet1p and Sec22p under oxidizing conditions. Moreover, this Bet1p-Sec22p heterodimer formed during in vitro transport reactions when ER-derived vesicles containing the Bet1p derivative fused with Golgi membranes containing the Sec22p derivative. Using this disulfide-crosslinking assay, we show that inhibition of transport with Sly1p antibodies blocked formation of the Bet1p-Sec22p heterodimer. In contrast, chelation of divalent cations did not inhibit formation of the Bet1p-Sec22p heterodimer during in vitro transport but potently inhibited Golgi-specific carbohydrate modification of glyco-pro-alpha factor. These results suggest that Ca2+ is not directly required for membrane fusion between ER-derived vesicles and Golgi acceptor membranes.