Abstract: Photometric study on the charge transfer reaction between phenytoin,the electron donor,and alizarin S,the electron acceptor,was made,and based on this charge transfer reaction,a photometric method for determination of sodium phenytoin (abbreviated as NaPTN) was proposed.In water solution,the charge transfer complex was formed between NaPTN and alizarin red with its absorption maximum at the wavelength 530 nm,and its molar ratio of 1 to 1.Apparent molar absorptivity (ε530) and stability constant of the complex were found to be 6.54×103L·mol-1·cm-1 and 2.26×105 respectively.Beer′s law was kept in the range of 4-40 mg·L-1 of NaPTN.Precision of the method was tested at the concentration level of 20 mg·L-1 for 10 multiplicate determinations,and the value of RSD found was 1.23%.The proposed method was applied to the determination of NaPTN in its tablets,giving results in consistency with the labelled value.Recovery of the method was tested by standard addition method,and values of recovery found were in the range of 97.7% to 101.0%.