Differently substituted benzimidazoles have been synthesized in very good yields in solvent-free conditions from o-phenylenediamine and aldehydes in the presence of BF3·OEt2 as a catalyst. The method is applicable to aromatic, unsaturated and aliphatic aldehydes and to substituted o-phenylenediamines without significant differences. Differently substituted benzimidazoles have been synthesized in very good results in solvent-free conditions from o-phenylenediamine and aldehydes in the presence of BF3 · OEt2 as a catalyst. The method is applicable to aromatic, unsaturated and aliphatic aldehydes and to substituted o-phenylenediamines without significant differences.