A derivative spectrophotometric procedure for the simultaneous determination of vanillin (4-hydroxy-3-methoxybenzaldehyde) and syringaldehyde (4-hydroxy-3,5-dimethoxybenzaldehyde) in rum is described. The method minimises the mutual interference between vanillin and syringaldehyde and allows the determination of these compounds without a previous extraction step. The precision of the method, expressed as the relative standard deviation, is better than 4%.