A highly sensitive analytical method was developed using GC/MS with temperature-programmable inlet on-column injection (TPI on-column GC/MS) for determining methyl dimethyldithiocarbamate (DMDC-methyl) and dimethyl ethylenebisdithiocarbamate (EBDC-dimethyl), which are methyl derivatives of alkali decomposed polycarbamate. This method makes it possible to quantify 0.3 microg/l of polycarbamate in tap water, which is a 1/100 of the residual target value of 30 microg/l in Japan. Moreover, it now becomes possible to distinguish polycarbamate from other dithiocarbamate pesticides (DTCs) that have similar structures, including ziram and thiram, which only incorporate a DMDC side chain, or manzeb, maneb and zineb, which only incorporate an EBDC side chain, by simultaneously analyzing for DMDC-methyl and EBDC-dimethyl.