Abstract: Known organic solderability preservatives (OSP) contain an azole compound and are applied on a copper surface from an acidic solution. The OSPs are not visible to the naked eye when applied. A number of dyes compatible with the OSP solution and not interfering with solderability has been identified. The composition containing such a dye can produce a layer which is visible to the naked eye when applied even in a thin layer. The suitable dyes contain a cationic imine group having the general formula ##STR1## where X.sup.- is an anion such as chloride, iodide, sulphate, etc., or an organically bonded anionic group such as sulphonate.