The traditional method for in vitro folding of disulfide-containing proteins is slow and involves a redox buffer of glutathione and glutathione disulfide. To increase the folding rate and to gain insight into the folding process, we replaced glutathione, an aliphatic thiol, with a commercially available aromatic thiol, 4-mercaptobenzeneacetate (1). Aromatic thiol 1 was selected due to its enhanced nucleophilicity and its enhanced leaving-group ability relative to glutathione at pH 7.7. To demonstrate the advantages of 1, the folding of reduced and scrambled RNase A at pH 7.0 and 7.7 in the presence of 1 and glutathione was investigated. For each set of folding conditions, the optimum concentration of each thiol was initially determined and then the folding rates in the presence of each thiol were measured concurrently. In all cases examined, the folding rate enhancement with the aromatic thiol was 5- 6-fold. Furthermore, under similar conditions folding rates were almost identical with either reduced or scrambled RNase A. In addition the 5-6-fold folding rate enhancement varied only slightly with pH, 7.0 vs 7.7.