Abstarct: In this thesis, two novel, simple, and accurate spectrophotometric methods are presented for the simultaneous determination of methyl violet and toluidine blue dyes. The spectrophotometric methods used including the extended ratio subtraction method (EXRMS) and the simultaneous ratio subtraction method (SRSM) were compared with mean centering of ratio spectral method (MCM). All the factors affecting the sensitivity of the methods were examined and optimized. The linear ranges for methyl violet and toluidine blue in the proposed spectrophotometric methods (SRSM and ERMS) were 1.00-10.00 and 1.00-15.00 μg mL-1, respectively, and for MCM, they were, respectively, 1.00-10.00 and 3.00-15.00 μg mL-1. For the EXRMS, SRSM, and MCM spectrophotometric methods, the limits of detection for methyl violet were found to be 0.010, 0.024, and 0.056 μg mL-1, respectively, and for toluidine blue, they were 0.032, 0.030, and 0.0281 μg mL-1, respectively. The effect of interference of different materials on the selectivity of the methods was also examined. The proposed methods were also applied to the simultaneous determination of methyl violet and toluidine blue in the water samples taken from the city of Shahrood (Iran) with satisfactory results. The results obtained indicated that the proposed methods had good accuracies and precisions for the simultaneous determination of these dyes in the real samples. Abstract 2 In this thesis, two novel, simple, and accurate spectrophotometric methods are presented for the simultaneous determination of the venlafaxine and citalopram drugs. The spectrophotometric methods used including the extended ratio subtraction method (EXRMS) and the simultaneous ratio subtraction method (SRSM) were compared with mean centering of ratio spectral method (MCM). All the factors affecting the sensitivity of the methods were examined and optimized. The linear ranges for venlafaxine and citalopram in the proposed spectrophotometric methods (SRSM and ERMS) were 1.60-20.00 and 1.00-16.00 μg mL-1, respectively, and for MCM, they were 2.00-20.00 and 3.00-16.00 μg mL-1, respectively. For the EXRMS, SRSM, and MCM spectrophotometric methods, the limits of detection for venlafaxine were found to be 0.171, 0.1399, and 0.437 μg mL-1, respectively, and for citalopram, they were 0.0101, 0.0101, and 0.04028 μg mL-1, respectively. The effect of the interference of different materials on the selectivity of the methods was also examined. The proposed methods used for the simultaneous determination of venlafaxine and citalopram were also applied to their tablet samples with satisfactory accuracy and correctness.