Acidity and basicity constants of N-acetylaspartic acid, aspartic acid, aspartylaspartic acid, Asp-Asp-Asp, Asp-Asp-Asp-Asp, Asp-Asp-Asp-Asp-Asp and Asp-Asp-asp-Asp-Asp-Asp were determined in 0.1 M potassium nitrate at 25 degree celsius, by means of potentiometric titration. Acidity constants of N-acetylaspartic acid are log K1 = -3.41 and log K2 = -5.13. Basicity constants, expressed as log K, of aspartic acid (asp) and polyaspartic acids (asp)n ; when n=2 to 6, are 9.80 and 8.79, 8.34, 8.50, 8.56 and 8.99 respectively. The number of acidity constant of polyaspartic acids (asp)n is equal to n+1 that is equivalent to the number of carboxylic acid in molecule. The pI’s of aspartic acid, aspartylaspartic acid, Asp-Asp-Asp, Asp-Asp-Asp-Asp, Asp-Asp-Asp-Asp-Asp และ Asp-Asp-Asp-Asp-Asp-Asp in aqueous solution of 0.1 M potassium nitrate at 25 degree celsius are 3.1, 3.3, 3.1, 2.8, 2.7 and 2.4 respectively.