The transformation mechanisms of thiourea in ethylene glycol solution was systematically investigated in this report, which shows the transformation process is influenced by the anion (NO3−, Cl−, Br−) and polyvinylpyrrolidone (PVP). Thiourea (tu) isomerizes into ammonium thiocyanate when NO3− is present, regardless of the existence of PVP. For Cl−, thiourea coordinates with copper anion to form [Cu(tu)]Cl·1/2H2O complex whether PVP is present. When it comes to Br−, thiourea hydrolyzes in the cooperation of PVP or coordinates with copper anion to form [Cu(tu)Br]·1/2H2O complex without PVP. The different transformation routes will lead to different phase evolution of the CuS system. This work may provide a new understanding of the transformation of thiourea in ethylene glycol solution. The optical properties of the as-prepared copper sulfides exhibit signi?cant stoichiometry-dependent features which may have potential applications in semiconductor photovoltaic devices.