A method of forming a metal layer on an electrically insulating substrate comprises depositing a photocatalyst layer onto the substrate and depositing a mask layer comprising voids on the substrate, such as a layer of latex microparticles with voids between them, to give an open pore structure to the mask. An electroless plating solution is then provided on the photocatalyst layer, and the photocatalyst layer and electroless plating solution are illuminated with actinic radiation whereby deposition of metal from the electroless plating solution to form a metal layer on the photocatalyst layer is initiated whereby the metal deposits in the voids of the mask layer. The mask layer is subsequently removed to leave a porous metal layer on the substrate. The method allows for deposition of porous metal films with controlled thickness and excellent adhesion onto electrically insulating substrates. The method is suitable for providing metal layers with controlled, regular porosity.