This work discusses the optimum conditions for quantitative analysis of two-component solutions if both components absorb at the wavelength at which the determination is made. Two conditions must be met for highest accuracy. (1) the solution as a whole should transmit about 40% of the incident energy. (2) When this condition is fulfilled, the interfering component should absorb as little as possible. In differential analysis with double-beam spectrophotometers the solution is placed in one beam and a known similar mixture is placed in the other. Increasing accuracy by changing the solutions in the beams is described. Constant slit widths are very important. This is more sensitive than single-beam methods.