Use of ureas in which at least one of the two nitrogen atoms of the urea is part of a non-aromatic ring which can be interrupted by an oxygen or sulphur atom, as a chemical solvent, and a process for the preparation of tri- or tetraalkylated ureas or diureas, at least one of the nitrogen atoms of the urea being part of a non-aromatic ring which can be interrupted by an oxygen or sulphur atom, by dialkylation of the -NH2 group of a urea with an alkylene- or alkylenearylenealkylene dihalide, disulphonate or dihydrogensulphate in the presence of a solid base and of a phase-transfer catalyst.