Abstarct: In this thesis, four new spectrophotometric methods including the constant center (CC) method, ratio difference (RD) method, ratio H-point standard addition method (RHPSAM), and H-point standard addition method (HPSAM) were studied. The aim was to select the best method for determining the species with a severe spectral overlapping. In this work, two pairs of species with severe spectral overlapping were studied separately; catechol (CAT) and resorcinol (RES) were studied in one mixture and the para- and mexta-nitroaniline isomers were studied in another one; their determination required separation. It was not possible to determine a species in a binary mixture by the simple spectrophotometric methods, and their separation was required. A comparative study of the results obtained showed that out of the four methods studied, the CC and RD methods had the least error in determining the concentration of the species with a high spectral overlapping. For both the CC and RD methods, the linear dynamic range for both CAT and RES was 2.5-20.0 mg/L; the detection limit for CAT was 0.582 and 0.292 mg/L, respectively, and for RES, It was 0.89 and 0.355 mg/L, respectively. For both the CC and RD methods, the linear dynamic range for mexta-nitroaniline was obtained to be 5.00-60.0 mg/L; for para-nitroaniline, it was 1.00-10.0 mg/L. For the CC and RD methods, the detection limit for mexta-nitroaniline was obtained to be 0.475 mg/L and 0.498 mg/L, respectively, and for para-nitroaniline, it was 0.047 mg/L and 0.048 mg/L, respectively.