Process for the production of sulphuric acid and/or oleum comprising the steps of: (a) producing a feed gas containing 5-50 mol % of SO 2 and a molar concentration of H 2 O being 50-150% of the molar concentration of SO 2: (b) passing the feed gas through a first SO 2 -conversion step in which SO 2 is oxidized to SO 3 in one or more catalyst beds; (c) cooling the SO 3 -containing gas from said first SO 2 -conversion step to a temperature 0-100°C above the sulphuric acid dew point of the gas; (d) passing the gas to an intermediate sulphuric acid condensing stage wherein the SO 3 -containing gas is cooled and sulphuric acid is condensed in air cooled tubes in which the SO 3 -gas flows downwards while the cooling air flows counter currently upwards the intermediate condenser and in which said air is provided from air recycling loop adapted to said intermediate condenser, and withdrawing from the bottom of the intermediate condenser a stream of condensed sulphuric acid or oleum, and a gas stream containing unconverted SO2 and uncondensed SO 3 and H 2 SO 4; (e) providing water and oxygen to the gas stream from the intermediate condenser containing unconverted SO 2 and uncondensed SO 3 and H 2 SO 4 by adding to this gas stream air withdrawn from said air recycling loop, in which the air recycling loop comprises: cooling of the air, adding water to the air by evaporating water in a humidifier, providing air to the air-recycling loop; heating the air of step and by passing the air through the intermediate condenser, withdrawing a portion of air which has been heated according to step and adding this air to said gas stream from the intermediate condenser containing unconverted SO 2 and condensed SO 3 and H 2 SO 4; (f) reheating the resulting gas stream from step (e) and passing this gas to a second SO 2 -conversion step in which remaining SO 2 is oxidized to SO 3 in one or more catalyst beds, cooling the gas to a temperature 0-100°C above its H 2 SO 4 dew point and subsequently passing the gas to a final condensing stage in which the remaining sulphuric acid is condensed by cooling of said gas in a final condenser containing air-cooled vertical glass tubes in which the gas flows upwards while the air flows counter-currently downwards, and withdrawing from said final condenser a stream of sulphuric acid; (a) producing a feed gas containing 5-50 mol % of SO 2 and a molar concentration of H 2 O being 50-150% of the molar concentration of SO 2: (b) passing the feed gas through a first SO 2 -conversion step in which SO 2 is oxidized to SO 3 in one or more catalyst beds; (c) cooling the SO 3 -containing gas from said first SO 2 -conversion step to a temperature 0-100°C above the sulphuric acid dew point of the gas; (d) passing the gas to an intermediate sulphuric acid condensing stage wherein the SO 3 -containing gas is cooled and sulphuric acid is condensed in air cooled tubes in which the SO 3 -gas flows downwards while the cooling air flows counter currently upwards the intermediate condenser and in which said air is provided from air recycling loop adapted to said intermediate condenser, and withdrawing from the bottom of the intermediate condenser a stream of condensed sulphuric acid or oleum, and a gas stream containing unconverted SO2 and uncondensed SO 3 and H 2 SO 4; (e) providing water and oxygen to the gas stream from the intermediate condenser containing unconverted SO 2 and uncondensed SO 3 and H 2 SO 4 by adding to this gas stream air withdrawn from said air recycling loop, in which the air recycling loop comprises: cooling of the air, adding water to the air by evaporating water in a humidifier, providing air to the air-recycling loop; heating the air of step and by passing the air through the intermediate condenser, withdrawing a portion of air which has been heated according to step and adding this air to said gas stream from the intermediate condenser containing unconverted SO 2 and condensed SO 3 and H 2 SO 4; cooling of the air, adding water to the air by evaporating water in a humidifier, providing air to the air-recycling loop; heating the air of step and by passing the air through the intermediate condenser, withdrawing a portion of air which has been heated according to step and adding this air to said gas stream from the intermediate condenser containing unconverted SO 2 and condensed SO 3 and H 2 SO 4; (f) reheating the resulting gas stream from step (e) and passing this gas to a second SO 2 -conversion step in which remaining SO 2 is oxidized to SO 3 in one or more catalyst beds, cooling the gas to a temperature 0-100°C above its H 2 SO 4 dew point and subsequently passing the gas to a final condensing stage in which the remaining sulphuric acid is condensed by cooling of said gas in a final condenser containing air-cooled vertical glass tubes in which the gas flows upwards while the air flows counter-currently downwards, and withdrawing from said final condenser a stream of sulphuric acid;