Densities, partial molar volumes, and viscosities of aqueous solutions of betaine have been measured at 5°, 10°, 15°, 20°, 25°, 30°, 37°, and 45° over the concentration range 0.05 to 5.0 mol/L-1.The partial molar volumes show that betaine exists partly as a monohydrate and partly in its anhydrous form.The proportion of the anhydrous form increases with increasing temperatureAlso, an associated form of betaine appears in concentrated betaine solutions, possibly with water as a bridging group.The significance of the viscosity B-coefficient is discussed.The signs of Bst, the increment of the viscosity B-coefficients arising from structural changes of water, are neg. and the signs of dB/dT, the temperature derivative of B, are pos.These results show that betaine is a water structure breaker especially at lower temperatures, and this effect decreases to insignificance at higher temperaturesThe ionization equilibrium of betaine were investigated in aqueous 0.5 mol/L-1 and 1.0 mol/L-1 NaNO3 at 5°, 15°, 25°, and 37° by a potentiometric method.Using the least-square computer program SUPERQUAD, the complex forms are deduced to be betainium BH, bis(betainium) BHB, and bis(betaine) B2, or bis(betaine)hydrate BH2OB.