In a stainless-steel beaker of approximately 3-l. volume (180 mm. by 150 mm.) (Note 1), equipped with an efficient mechanical stainless-steel stirrer and heated by an electric hot plate, are placed 84 g. (2 moles) of 97% sodium hydroxide pellets, 332 g. (5 moles) of 85% potassium hydroxide pellets (Note 2), and 50 ml. of water. The mixture is stirred and heated. When the temperature of the fluid mixture reaches 160°, 152 g. (1 mole) of vanillin is added in portions over a period of 2.5–3 minutes at a rate sufficient to maintain the reaction (Note 3). The temperature at this point is 190–195°. Stirring is continued, and heat is applied until the temperature reaches 240–245° (Note 4). The temperature is maintained at 240–245° for 5 minutes. The hot plate is removed, and the mixture is allowed to cool with stirring. When the mixture has cooled to about 150–160°, 1 l. of water is added, and the mixture is stirred until all the fusion mixture is dissolved. The solution is transferred to a 4-l. beaker, another 500 ml. of water is added, and sulfur dioxide gas is introduced for 2 minutes (Note 5); the mixture is then completely acidified with 1.5 l. of 6 N hydrochloric acid. The acidified mixture is cooled in an ice bath (5°) for 2 hours, and the crystalline precipitate is filtered, washed on the filter with two 100-ml. portions of ice water, and air dried. The tan crystals of protocatechuic acid melting at 196–198° weigh 90–100 g. Extraction of the filtrate and washings with two 1-l. portions of ether yields an additional 45–55 g. of protocatechuic acid melting at 190°. The total yield of crude protocatechuic acid amounts to 135–153 g. (89–99%) (Note 6).