Aqueous ethylene glycol (EG) mixtures are decomposed by film boiling at near saturation temperatures on a horizontal tube in a stagnant pool containing up to 20% (volume) water. The reactor volume is the vapor layer that blankets the tube in the film boiling regime. Chemical reactions are promoted within the vapor film by the tube temperatures while the bulk liquid is close to its bubble point temperature. Experiments are carried out on bare tubes and tubes coated with nickel and platinum catalysts to show the effects involved. Results show that chemical conversion of the hydrocarbon vapors produces primarily CO and H-2. Product yields (flow rates) are enhanced on a catalyst, with an 80%EG/20%water mixture (volume percent) showing three to four times higher product yields compared to a bare tube. Platinum coatings showed slightly higher yields than nickel coatings. Diluting ethylene glycol with water decreases the overall chemical reactivity owing to preferential vaporization of water that enriches the film with steam. The presence of steam in the vapor film appears to reduce carbon deposition or "coking" on the tube when enrichment by steam is significant: deposits were observed for pure EG and 90%EG/10%water mixtures but not for 80%EG/20%water mixtures. (C) 2012 Elsevier Ltd. All rights reserved.