Aqueous alkaline composition free from organic solvents and metal ion-free silicates, the said compositions comprising (A) a thioamino acid having at least one primary amino group and at least one mercapto group, (B) a quaternary ammonium hydroxide, (C) a chelating and/or corrosion inhibiting agent selected from the group consisting of aliphatic and cycloaliphatic amines having at least two primary amino groups, and aliphatic and cycloaliphatic amines having at least one hydroxy group, (D) a nonionic surfactant selected from the group of acetylenic alcohols, alkyloxylated acetylenic alcohols and alkyloxylated sorbitan monocarboxylic acid mono esters; the use of the alkaline composition for the processing of substrates useful for fabricating electrical and optical devices; and a method for processing substrates useful for fabricating electrical and optical devices making use of the said aqueous alkaline composition.