Thermal control in synthesis of hydrocarbons with more than 1 C atom per mol. from H2 and an oxide of C is achieved by introducing liquid Hg into the upper portion of a vertical reaction chamber. The Hg flows downward over the catalyst in direct heat exchange with the reacting gases. When the excess exothermic heat of reaction is removed primarily by removal as sensible heat of the liquid Hg, it is preferable to have countercurrent flow of gases and Hg. When the temperature is so high that a large portion of the Hg will be vaporized, thus removing the exothermic heat as latent heat of vaporization of the Hg, it is preferable to introduce the synthesis gas near the upper portion of the reaction chamber resulting in concurrent flow of reacting gases and Hg. 1 foreign patent and 6 United States patents cited.