A process for isolating 2,4-bis-(2,4-dihydroxyphenyl)-6-(4-methoxyphenyl)-1,3,5-triazine (DIOPAT) from an aqueous alkaline mixture M having a pH of 10 or more and comprising (i) the 2,4-bis-(2,4-dihydroxyphenyl)-6-(4-methoxyphenyl)-1,3,5-triazine; (ii) 2,4-dihydroxybenzophenone; (iii) aluminum salts; wherein the process comprises the steps of a) separating the 2,4-bis-(2,4-dihydroxyphenyl)-6-(4-methoxyphenyl)-1,3,5-triazine from the aluminum salts and the 2,4-dihydroxybenzophenone by nanofiltration of the alkaline mixture M, wherein the 2,4-bis-(2,4-dihydroxyphenyl)-6-(4-methoxyphenyl)-1,3,5-triazine is obtained in the form of an alkaline aqueous solution S as the retentate; b) precipitating the 2,4-bis-(2,4-dihydroxyphenyl)-6-(4-methoxyphenyl)-1,3,5-triazine by modifying the pH of the aqueous solution S to a value of 9.5 or lower; c) separating the precipitated 2,4-bis-(2,4-dihydroxyphenyl)-6-(4-methoxyphenyl)-1,3,5-triazine from the aqueous solution S by filtration, wherein the 2,4-bis-(2,4-dihydroxyphenyl)-6-(4-methoxyphenyl)-1,3,5-triazine is obtained in the form of an aqueous suspension SP as the retentate; wherein the nanofiltration step a) is performed at a pressure of from 10 to 30 bar and at a temperature in the range of from 20 to 60 °C; and wherein the nanofiltration step a) is performed with a nanofiltration membrane having a sodium chloride retention rate of from 60 % to 75 %, wherein the sodium chloride retention rate is measured at 25 °C at 10 bar with a starting concentration of sodium chloride of 500 ppm and with 10-15 % permeate yield. (i) the 2,4-bis-(2,4-dihydroxyphenyl)-6-(4-methoxyphenyl)-1,3,5-triazine; (ii) 2,4-dihydroxybenzophenone; (iii) aluminum salts; wherein the process comprises the steps of a) separating the 2,4-bis-(2,4-dihydroxyphenyl)-6-(4-methoxyphenyl)-1,3,5-triazine from the aluminum salts and the 2,4-dihydroxybenzophenone by nanofiltration of the alkaline mixture M, wherein the 2,4-bis-(2,4-dihydroxyphenyl)-6-(4-methoxyphenyl)-1,3,5-triazine is obtained in the form of an alkaline aqueous solution S as the retentate; b) precipitating the 2,4-bis-(2,4-dihydroxyphenyl)-6-(4-methoxyphenyl)-1,3,5-triazine by modifying the pH of the aqueous solution S to a value of 9.5 or lower; c) separating the precipitated 2,4-bis-(2,4-dihydroxyphenyl)-6-(4-methoxyphenyl)-1,3,5-triazine from the aqueous solution S by filtration, wherein the 2,4-bis-(2,4-dihydroxyphenyl)-6-(4-methoxyphenyl)-1,3,5-triazine is obtained in the form of an aqueous suspension SP as the retentate; a) separating the 2,4-bis-(2,4-dihydroxyphenyl)-6-(4-methoxyphenyl)-1,3,5-triazine from the aluminum salts and the 2,4-dihydroxybenzophenone by nanofiltration of the alkaline mixture M, wherein the 2,4-bis-(2,4-dihydroxyphenyl)-6-(4-methoxyphenyl)-1,3,5-triazine is obtained in the form of an alkaline aqueous solution S as the retentate; b) precipitating the 2,4-bis-(2,4-dihydroxyphenyl)-6-(4-methoxyphenyl)-1,3,5-triazine by modifying the pH of the aqueous solution S to a value of 9.5 or lower; c) separating the precipitated 2,4-bis-(2,4-dihydroxyphenyl)-6-(4-methoxyphenyl)-1,3,5-triazine from the aqueous solution S by filtration, wherein the 2,4-bis-(2,4-dihydroxyphenyl)-6-(4-methoxyphenyl)-1,3,5-triazine is obtained in the form of an aqueous suspension SP as the retentate; wherein the nanofiltration step a) is performed at a pressure of from 10 to 30 bar and at a temperature in the range of from 20 to 60 °C; and wherein the nanofiltration step a) is performed with a nanofiltration membrane having a sodium chloride retention rate of from 60 % to 75 %, wherein the sodium chloride retention rate is measured at 25 °C at 10 bar with a starting concentration of sodium chloride of 500 ppm and with 10-15 % permeate yield.