The question of the order of kinetics of the thermoluminescence (TL) of peak 5 in LiF:Mg,Ti has been subject to considerable controversy. Recently, we have shown that peak shape analysis of post-irradiation annealed LiF:Mg,Ti (TLD-100) at 165 leads to non-first-order kinetics, reaching a value as high as following a 165 30 min post-irradiation anneal. When no post-irradiation anneal is applied, peak shape analysis almost always yields first-order behaviour. Isothermal decay data are equally ambiguous. To begin to resolve this discrepancy (at least at the experimental level) we have studied the isothermal decay of peak 5 at 165 (presumably cleansed of the low-temperature peaks 2 - 4 by a 165 15 min post-irradiation anneal). The isothermal decay was initiated at four dose levels of 10, 100, 300 and 1000 Gy, and these decays were not even vaguely similar. The initial rate of decay at Gy was at least five times greater than at 10 Gy and the decays were clearly non-exponential. Our data are not consistent with first-order decay (b = 1) and lead to using general-order (GO) model analysis.