Guaiacol is a naturally occurring organic compound with the formula C6H4(OH)(OCH3), first isolated by Otto Unverdorben in 1826. Although it is biosynthesized by a variety of organisms, this yellowish aromatic oil is usually derived from guaiacum or wood creosote. Samples darken upon exposure to air and light. Guaiacol is present in wood smoke, resulting from the pyrolysis of lignin. The compound contributes to the flavor of many compounds, e.g., roasted coffee.

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