The invention relates to a process for preparing nitric acid by treating an aqueous medium containing organic compounds and phosphate, said process comprising: feeding the aqueous medium to a nitric acid synthesis zone; forming nitric acid by contacting the aqueous medium with a gaseous medium in said nitric acid synthesis zone, said gaseous medium containing NO.sub.2; discharging an off-gas from said nitric acid synthesis zone; wherein the total organic carbon concentration in the aqueous medium entering the nitric acid synthesis zone is less than 0.03 wt. %.