Treatment of the diazonium salts, X•C6H4N2+, with aqueous methylamine affords good yields of the monomethyltriazenes, X•C6H4•N=N•NHMe, when the substituent is a strongly electron-withdrawing group (X = o-,m-, andp-NO2;o-,m-, andp-CO2R;p-CN andp-COCH3). Preparation of the triazene from thep-bromobenzene diazonium salt was accompanied by formation of a pentaazadiene. Monomethyltriazenes were not obtained when diazonium salts containing other substituents (X = H,p-CH3,o-CF3,p-Cl,p-F,p-NMe2,p-OH,p-OCH3,p-Ph,p-NHCOCH3) were treated with methylamine. In these cases the products were either pentaazadiene, or 1,3-diaryltriazenes or unstable materials. The monomethyltriazenes vary considerably in stability and give rise to a number of different degradation products, which were either diaryltriazenes or 3-alkyl-1,3-diaryltriazenes or simply arylamines. 1(3)-(p-Nitrophenyl)-3(1)-methyltriazene was found to be a moderately effective methylating agent.