When ordinary laboratory Schiff's reagent is mixed with solutions of certain aromatic aldehydes a yellow precipitate is formed instead or the usual violet colouration. The reaction of almost 50 aldehydes to Schiff's reagent is given and this abnormal precipitation is shown to tare place only with aromatic ortho-hydroxy aldehydes with two exceptions in the cases of cinnan-aldehyde and ìndole aldehyde. Excess of SO₂ favours the normal reaction and inhibits the precipitation or the yellow compounds The precipitates are amorphous insoluble mixtures and are hydrated Schiff's bases which are produced by the condensation of the pararosaniline-c-leueosulphonic acid of the Schiff's reagent with the aldehyde. A theory is put forward to explain the abnormal reaction. The colouration of Schiff's reagent which results when the reagent is heated has been examined in detail and an explanation put forward.