In-situ combustion is the most energy-efficient of thermal enhanced oil recovery methods. In light-oil reservoirs, too little fuel may be deposited and in very heavy-oil reservoirs, too much fuel may be deposited. A research program has been ongoing to try to solve these problems. Water-soluble additives were tested in an attempt to modify the fuel deposition reactions. In the first stage, kinetics experiments were run on Huntington Beach (18.5 degrees API) and Hamaca, Venezuela (10.5 degrees API) oils in the presence of aqueous solutions of metallic salts. The results were compared with control runs with no metal present. While copper, nickel, and cadmium salts had little or no effect, iron, tin, zinc, and aluminum increased fuel laydown for Huntington Beach oil, with similar results for the heavier Hamaca oil. As no reduction in fuel was noticed with any of the metallic additives, ketals (ketyl acetates) were mixed with Hamaca oil in an unsuccessful attempt to reduce fuel. Twenty-one combustion-tube runs were made in the second stage of the study. They included runs with the oils previously tested in the kinetics apparatus plus two California oils, Cymric light (36 degrees API) and Cymric heavy (12 degrees API), as well as Saudi ＇＇tar＇＇(19.7 degrees API). Iron, tin, and zinc salts improved the combustion efficiency in all cases. As a result of the additives, the frontal velocities were increased. Changes were also observed in the H/C ratios of the fuel, heats of combustion, air requirements and density of the crude produced. The amount of fuel deposited varied among the oils. For Huntington Beach oil, the amount of fuel increased in the order zinc, control, tin, and iron, while for the Hamaca crude the order was control, iron, and tin. Cymric heavy oil showed the order control, zinc, and iron. Cymric light oil and the Saudi tar were tested only with iron additive. In the last case the effect of additive concentration was also investigated. We found that for this specific case, a minimum near 5% by weight of hydrated ferrous nitrate in the connate water was needed to improve the combustion. To date no suitable additive has been found to reduce fuel deposition. Iron and tin salts seem suitable agents to increase fuel when that is needed.