A process for preparing acetylene and synthesis gas by partial oxidation of hydrocarbons with oxygen, in which a first input stream comprising one or more hydrocarbons and a second input stream comprising oxygen - are separately preheated, - mixed in a ratio of the mass flow rates of the second input stream to the first input stream corresponding to an oxygen ratio λ of less than or equal to 0.35, oxygen ratio λ being understood to mean the ratio of the amount of oxygen actually present in the second input stream to the stoichiometrically necessary amount of oxygen required for the complete combustion of the one or more hydrocarbons present in the first input stream, - supplied via a burner block to a combustion chamber in which the partial oxidation of the hydrocarbons takes place, - to obtain a cracking gas which is quenched to 200°C to 250°C downstream of the combustion chamber by injection of a quench oil, to obtain - a product gas stream I g which - is cooled with further quench oil in a burner column, by drawing off liquid from one or more suitable stages in the burner column, cooling it by indirect heat exchange with water to raise steam, and supplying it again to the burner column above the stage from which it was drawn off, - to obtain a product gas stream II g which has been cooled to 60°C to 90°C and which - is passed into a final cooler in which direct heat exchange with water gives a product gas stream III g cooled to 20°C to 50°C, and a process water stream I liq, - which comprises subjecting the process water stream I liq to a cleaning operation by partial vaporization in a one-stage flash vessel, the process water stream I liq being vaporized in a proportion of 0.01% by weight to 10% by weight, based on the total weight thereof, to obtain a cleaned process water stream II liq which is disposed of in the wastewater. - are separately preheated, - mixed in a ratio of the mass flow rates of the second input stream to the first input stream corresponding to an oxygen ratio λ of less than or equal to 0.35, oxygen ratio λ being understood to mean the ratio of the amount of oxygen actually present in the second input stream to the stoichiometrically necessary amount of oxygen required for the complete combustion of the one or more hydrocarbons present in the first input stream, - supplied via a burner block to a combustion chamber in which the partial oxidation of the hydrocarbons takes place, - to obtain a cracking gas which is quenched to 200°C to 250°C downstream of the combustion chamber by injection of a quench oil, to obtain - a product gas stream I g which - is cooled with further quench oil in a burner column, by drawing off liquid from one or more suitable stages in the burner column, cooling it by indirect heat exchange with water to raise steam, and supplying it again to the burner column above the stage from which it was drawn off, - to obtain a product gas stream II g which has been cooled to 60°C to 90°C and which - is passed into a final cooler in which direct heat exchange with water gives a product gas stream III g cooled to 20°C to 50°C, and a process water stream I liq, - which comprises subjecting the process water stream I liq to a cleaning operation by partial vaporization in a one-stage flash vessel, the process water stream I liq being vaporized in a proportion of 0.01% by weight to 10% by weight, based on the total weight thereof, to obtain a cleaned process water stream II liq which is disposed of in the wastewater.