A process for obtaining formic acid by thermal separation of a stream comprising formic acid and a tertiary amine which at a pressure of 1013 hPa abs has a boiling point which is at least 5°C higher than that of formic acid and which has the general formula NR 1 R 2 R 3 in which the radicals R 1 to R 3 are selected independently from the group consisting of C 5 -C 8 -alkyl, in which (a) a liquid stream comprising formic acid, tertiary amine and water is produced by combining tertiary amine and a formic acid source in the presence of water, where (i) a formic acid source comprising methyl formate is used and the liquid stream comprising formic acid, tertiary amine and water is obtained therefrom by hydrolysis of methyl formate; or (ii)a formic acid source which comprises carbon dioxide is used and the liquid stream comprising formic acid, tertiary amine and water is obtained therefrom by transition metal-catalyzed hydrogenation of carbon dioxide, and the liquid stream comprising formic acid, tertiary amine and water has a molar ratio of formic acid to tertiary amine of from 0.5 to 5; (b) water and organic decomposition products of the tertiary amine are separated off from the liquid stream obtained from step (a), with the organic decomposition products of the tertiary amine having been comprised in the tertiary amine fed to step (a) and/or been formed during the course of the process up to the present step (b), and a liquid stream which comprises formic acid and tertiary amine and is depleted in water and organic decomposition products of the tertiary amine is obtained; and (c) formic acid is removed by distillation from the liquid stream comprising formic acid and tertiary amine obtained from step (b) in a distillation apparatus at a temperature at the bottom of from 100 to 300°C and a pressure of from 30 to 3000 hPa abs; wherein the stream comprising water and organic decomposition products of the tertiary amine which has been separated off in step (b) is separated into two liquid phases; the upper liquid phase enriched in organic decomposition products of the tertiary amine is removed; and the lower, water-comprising liquid phase is recirculated to step (a). (a) a liquid stream comprising formic acid, tertiary amine and water is produced by combining tertiary amine and a formic acid source in the presence of water, where (i) a formic acid source comprising methyl formate is used and the liquid stream comprising formic acid, tertiary amine and water is obtained therefrom by hydrolysis of methyl formate; or (ii)a formic acid source which comprises carbon dioxide is used and the liquid stream comprising formic acid, tertiary amine and water is obtained therefrom by transition metal-catalyzed hydrogenation of carbon dioxide, and the liquid stream comprising formic acid, tertiary amine and water has a molar ratio of formic acid to tertiary amine of from 0.5 to 5; (i) a formic acid source comprising methyl formate is used and the liquid stream comprising formic acid, tertiary amine and water is obtained therefrom by hydrolysis of methyl formate; or (ii)a formic acid source which comprises carbon dioxide is used and the liquid stream comprising formic acid, tertiary amine and water is obtained therefrom by transition metal-catalyzed hydrogenation of carbon dioxide, and the liquid stream comprising formic acid, tertiary amine and water has a molar ratio of formic acid to tertiary amine of from 0.5 to 5; (b) water and organic decomposition products of the tertiary amine are separated off from the liquid stream obtained from step (a), with the organic decomposition products of the tertiary amine having been comprised in the tertiary amine fed to step (a) and/or been formed during the course of the process up to the present step (b), and a liquid stream which comprises formic acid and tertiary amine and is depleted in water and organic decomposition products of the tertiary amine is obtained; and (c) formic acid is removed by distillation from the liquid stream comprising formic acid and tertiary amine obtained from step (b) in a distillation apparatus at a temperature at the bottom of from 100 to 300°C and a pressure of from 30 to 3000 hPa abs; wherein the stream comprising water and organic decomposition products of the tertiary amine which has been separated off in step (b) is separated into two liquid phases; the upper liquid phase enriched in organic decomposition products of the tertiary amine is removed; and the lower, water-comprising liquid phase is recirculated to step (a).