Diffusion parameters of thermal neutrons in water were determined from diffusion length measurements in 22 boron-poisoned aqueous solutions. A uniform plane source of thermal neutrons and a parallelepipedal polymethyl methacrylate container having a buckling of B2 = 9.1 × 10−2 cm−2 were used. The derived values of the diffusion parameters of thermal neutrons in water at 22 °C are These results are found to be in good agreement with Doming's recent calculations.