With the objective of establishing a design space for the simultaneous assessment of chlordiazepoxide and amitriptyline by RP-HPLC, a methodical design of experiments was carried out. The method was developed using Symmetry ODS C18 (4.6mm 250mm, 5m) HPLC column, a mobile phase consisting of Methanol: Acetonitrile (35:65v/v), a flow rate of 1 mL per minute, and UV detection at 228 nm for both chlordiazepoxide and amitriptyline. The method was found to be linear in the range of 6.0-14.0 g mL1 and 30.0-70.0 g mL1, with a correlation coefficient of 0.9999 in each case. The recovery values ranged from 99.94% to 100.09% and 99.26% to 100.63% for chlordiazepoxide and amitriptyline, respectively. Statistical analysis of experimental data was carried out using Design-Expert software employing the response-surface methodology, central composite design, and quadratic model. In order to optimize the method, perturbation, contour, 3D, and design-space plots were taken into consideration along with response factors like resolution and retention time. Through the QbD approach, we established the design space and demonstrated the resilience of the method under chromatographic conditions. Furthermore, it has been established that this