Abstract: The level of the nitrosamines which may be present in the unrefined trifluralin is reduced to a value below 0.5 ppm, by treating the unrefined trifluralin under agitation, at a temperature from 60.degree. to 90.degree. C., for a time between 10 minutes and 4 hours, with an aqueous solution containing from 15 to 50% by weight of hydrobromic acid in a ratio of 0.2 to 0.8 liters per kilogram of unrefined trifluralin, said aqueous solution of hydrobromic acid being added with an amount of sulfamic acid from 0.1 to 3 g per liter of solution and with an amount of bisulfite or an alkali metal corresponding to a content of sulphur dioxide from 0.3 to 1.2 g per liter of said hydrobromic acid aqueous solution.The so obtained refined trifluralin has a content of nitrosamines which is lower than 0.1 ppm.