A method of forming a diffusion barrier layer on a dielectric or semiconductor substrate by a wet process. The method includes the steps of treating the dielectric or semiconductor substrate with an aqueous pretreatment solution comprising one or more adsorption promoting ingredients capable of preparing the substrate for deposition of the diffusion barrier layer thereon; and contacting the treated dielectric or semiconductor substrate with a deposition solution comprising manganese compounds and an inorganic pH buffer (optionally, with one or more doping metals) to the diffusion barrier layer thereon, wherein the diffusion barrier layer comprises manganese oxide. Also included is a two-part kit for treating a dielectric or semiconductor substrate to form a diffusion barrier layer thereon.