The invention relates to an in-situ process for preparing an alkylphenol-aldehyde resin. The process comprises the step of providing a raw alkylphenol composition. The raw alkylphenol composition comprises one or more alkylphenol compounds and at least about 1 wt % phenol. Each alkylphenol compound has one or more alkyl substituents. The raw alkylphenol composition is reacted directly, without pre-purification, with one or more aldehydes to form an in-situ alkylphenol-aldehyde resin. The invention also relates to an in-situ alkylphenol-aldehyde resin formed from the in-situ process, and its use in a tackifier composition and rubber composition. The tackifier composition and rubber composition containing the in-situ alkylphenol-aldehyde resin show, inter alia, improved tack performance.