In a Reimer-Tiemann reaction for the preparation of a mixture of salicylaldehyde and p-hydroxybenzaldehyde, improved yield of the product and high selectivity of salicylaldehyde can be obtained when an alkali metal salt of a phenol is reacted with a reaction mixture comprised of chloroform and a slurry of an alkali metal hydroxide (preferably, in a fine powder state thereof) and an inert organic solvent and optionally, a catalytic amount of a surfactant.