The present invention provides a method for the production of glycolaldehyde with high specificity. The hydrous thermolysis consists of the spraying of aqueous sugar solutions containing from 25 to 80% of water but preferably 30 to 60% water, as a fine mist into a reactor held at the between 500 and 600.degree. C., but preferably between 520 and 560.degree. C. and the condensation of the resulting vaporous product in a surface condenser with optional heat recovery. The residence time of the vaporous product in the reactor should be in the range 0.1 5 seconds, but preferably in the range 0.5 to 2 seconds. Aldose monomeric sugars, preferably glucose (also known as dextrose), are preferred for use in the aqueous solution. The yield of glycolaldehyde in the condensed liquid is minimum 50% by weight of the sugar fed for glucose solutions.