A continuous process for the preparation in at least one esterification reactor of phthalate diesters utilizing phthalic acid or anhydride and a stoichiometric excess of at least one saturated aliphatic alcohol containing about 4 to about 10 carbon atoms comprises (a) reaction of phthalic acid or anhydride with said alcohol in a reactor zone, (b) purification of the diester in a purification zone in which the reaction product is neutralized with an alkali metal hydroxide, i.e., sodium hydroxide, to separate the diester and the unreacted phthalic acid or anhydride together with said saturated alcohol utilized in the reaction, said reaction product is separated into an organic phase and an aqueous waste phase. The organic phase is washed with water, steam stripped, treated with activated charcoal and filtered to purify the diester product desired, (c) the aqueous phase containing monoester is separated from said organic phase in step (b) is purified by filtration to remove solid impurities, carbon treated to remove color bodies, and acidified and extracted with the same alcohol used in the esterification to obtain an organic phase which can be recycled to the reaction zone of the process without causing discoloration of the desired diester product of the process.In the monoester purification process, acidification is accomplished utilizing a strong organic acid, i.e., sulfuric acid, prior thereto or simultaneously with extraction utilizing the same alcohol used in the esterification process. Purification of the aqueous waste streams produced in the process can be accomplished using activated charcoal to remove organic wastes and precipitation of inorganic salts to remove inorganic impurities produced in the process.