The thermal behavior of lavender and sage essential oils (EOs) and also that of their main components (camphor and 1,8-cineole) were studied by thermogravimetry (TG) at different scan rates under inert gas atmosphere up to about 140 °C. All the samples investigated undergo a single-step evaporation starting from ambient temperature without evidence of side decomposition processes. On the basis of the onset temperatures (Ton) extrapolated from the TG curves, the following increasing stability trend is assessed:
1,8-cineole < sage < lavender < camphor.
However, the difference of Ton values among the first three samples was found to be slight. To the aim of assessing a thermal stability scale among the samples, a kinetic analysis of evaporation was performed. A first approach was based on a simple model-fitting kinetic method, widely used in the past for studying evaporation of EOs. The results were compared with those obtained with a more reliable approach based on the integral isoconversional method by Kissinger–Akahira–Sunose in which the activation energy (Ea) versus reaction extent (α) plot of each sample was obtained. Because of the compensation effect, neither activation energy nor the pre-exponential factor, alone, could be considered to compare the stabilities. Lastly, the Arrhenius kinetic rate constant values at given temperatures within the experimental range were calculated, and camphor was found to be the more stable component, while lavender seems to vaporize faster than any other sample tested.