The optical absorption (OA) spectrum of LiF:Mg,Ti has been investigated by optical bleaching and I m –T stop pulsed annealing in steps of 3–4°C over the temperature range from 100°C to 300°C. In agreement with previous investigations, the 5.45eV OA band is identified as a likely candidate for the recombination stage competitor responsible for composite peak 5 supralinearity. An additional OA band at 5.7eV is also identified as a possible recombination stage competitor. First-order kinetic analysis yields an activation energy of 0.8±0.13eV for the 4.0eV band usually associated with peak 5, an energy which is similar in value to the binding energy of the Mg–Li vac trimer believed to form the peak 5 trapping structure. Dose response measurements result in dose filling constants of βTC=0.9±0.1×10−3Gy−1 and βCC=3.8±0.8×10−4Gy−1 for the 4.0 and 5.45eV OA bands, respectively. These dose filling constants deduced from OA measurements can be used in the framework of the unified interaction model (UNIM) to study the dose response supralinearity of composite dosimetric peak 5.