Application of carbon nanotube (CNT) film as a gas sensor is highly desired. Transient behavior of molecular adsorption provides adsorption and desorption constants; however, to date, few researchers have investigated it. Here, we focused on the transient behavior of electric conductance of the CNT film during adsorption. Adsorption of polar molecules showed predicable behavior, but non-polar molecules showed a sudden change of electric conductance that indicated excess adsorption. We analyzed the process based on Langmuir adsorption, and adsorption and desorption constants, which are considered to be physical properties; however, these parameters did not show good agreement for the same film. (C) 2017 Elsevier B.V. All rights reserved.