Red cabbage dye is a natural pigment used mainly as a food color. A class of compounds called anthocyanins attributes to this color. The pH of the red cabbage solution can also affect both its color and intensity. The objective of this study was to determine the ionization constant (pK(a)) of red cabbage color, the effect of pH and temperature on its stability in solution and evaluation of this natural color as a pH indicator in pharmaceutical system. Spectrophotometric method was used to determine its pK(a). The lambda(max) and absorbencies of the red cabbage color at different concentrations and pH were determined. The analytical wavelength (AW) is the wavelength at which the greatest difference in absorbencies between ionized and molecular species occurs was determined. The absorbencies of red cabbage solution (0.12% w/v) at different pH values ranging from 5.0 to 8.0 (with increments of 0.2), was measured at the AW of 612 nm. The resulted absorbencies ranged from 0.31 to 1.91 and were used to determine its pK(a). The pK(a) determined by this method was within a range of 6.8-7.2. Results from this study demonstrated that red cabbage dye could be used as a pH indicator in pharmaceutical formulations. In acidic condition, it has its original red color but at a basic pH its color changes to deep blue. This color is more stable at a low temperature and pH. Its ability to act as a pH indicator was further tested using chlorbutol solution as a model system.