In this work, we simultaneously determined the diffusion coefficient of toluene vapor in air by following the toluene evaporation in two different geometries: in a thin glass tube and a sessile drop on poly(tetrafluoroethylene) (Teflon) substrate in the same cell by inducing a quasi-steady liquid-vapor phase transition. A glass cell whose walls were lined with activated carbon was used in both experiments. Photography was applied to monitor the sessile drop evaporation, which occurred with constant contact angle mode (approximately 43degrees). When the restriction of evaporation space by the presence of the substrate surface [f(theta) factor] was considered in the sessile drop evaporation, the toluene vapor diffusion coefficient in air was found to be close in both methods. The diffusion coefficient was found to vary linearly with the temperature for both methods, and there is a 3.17 power dependence when the absolute temperatures are used. However, this figure is higher than the 1.75-2.00 power dependence, which is generally used for evaporating liquid vapors in the existing literature. In addition, an equation was derived showing the decrease of the square of the height of the sessile drop should vary linearly with time and was confirmed with the experimental results. Similarly, the decrease of the square of the contact radius of the drops was found to vary linearly with time in the experiments as expected from the theory, which was previously derived.