An organic reagent, sodium salt of N,N-dimethyldi-thiocarbamate (DMDC) was investigated as a depressant for the separation of lead activated marmatite from galena in the presence of diethyldithiocarbamate (DDTC). The flotation tests of single mineral showed that lead-activated marmatite could be depressed efficiently using DMDC as the depressant. UV–vis spectroscopy and ICP study confirmed that DMDC had a stronger capacity of complexing with lead ions or hydroxy complexes of lead. FTIR and adsorption measurement showed that the adsorption of DMDC on galena and marmatite was chemisorption. In addition, after pre-treated with DMDC, DDTC could co-adsorb on the galena surface, however, less DDTC species adsorbed on the marmatite surfaces.