The invention relates to a process for improving the stability of a base-modified alkylphenol-aldehyde resin (such as an amine-modified alkylphenol-aldehyde resin). The process comprises the step of reacting a base-modified alkylphenol-aldehyde resin with a salicylic acid to stabilize the base-modified alkylphenol-aldehyde resin. The invention also relates to a modified alkylphenol-aldehyde resin formed from the stabilizing process, and its use in a tackifier composition and rubber composition.