A mild and highly efficient method for intramolecular disulfide bond formation in peptides mediated by charcoal has been developed. Completion of the charcoal-assisted catalysis of disulfide bond formation took less than 6 h, testing a series of peptides with ring sizes varying from 2 to 17 amino acids. Kinetic studies revealed that the formation of small and large intramolecular disulfide cycles especially was accelerated significantly by the aid of charcoal as compared with dimethyl sulfoxide- and air-mediated cyclization. Oxygen adsorbed onto the charcoal surface is a prerequisite for disulfide bond formation. Thermodynamic studies showed that cyclization is accelerated by reduction of entropy of the peptides, most likely because of transient adsorption to the charcoal surface, thus resulting in a lower activation energy.