It has been found that diazomethane undergoes a facile 1,3-dipolar cycloaddition with both dithiobenzoate RAFT agents and the dithiobenzoate end-groups of polymers formed by RAFT polymerization. Thus, 2-cyanoprop-2-yl dithiobenzoate on treatment with diazomethane at room temperature provided a mixture of stereoisomeric 1,3-dithiolanes in near quantitative (>95%) yield. A low-molecular-weight RAFT-synthesized poly(methyl methacrylate) with dithiobenzoate end-groups underwent similar reaction as indicated by immediate decolourization and a quantitative doubling of molecular weight. Higher-molecular-weight poly(methyl methacrylate)s were also rapidly decolourized by diazomethane and provided a product with a bimodal molecular weight distribution. Under similar conditions, the trithiocarbonate group does not react with diazomethane.