Abstract This work presents a novel approach in gas detection by an original method of microwave transduction. The design of the sensor includes a coplanar grounded wave guide with a gas sensing material to study its sensitivity to ammonia in argon flux. The sensing material can play the role of the substrate or can be deposited as a thin layer on a microstrip structure used in electronics. Submitted to an electromagnetic excitation in microwave energies, the sensor response in the presence of a gas results in a specific modification of the reflected wave (real and imaginary parts). The goals of this study include an examination of the form of the sensitive material and its influence on the response of the microwave gas sensor. Two cases are considered: bulk or thin layer. In bulk case, the material plays the role of the substrate of the microstrip structure. In the second case, a thin layer is deposited on the sensor. We showed how, in the presence of ammonia, the reflected wave is related to its concentration. The response to the material–gas interaction depends on the excitation frequency. The parameter used as the sensor response is the ratio of the reflected wave on the incident wave at each frequency. The study deals with the influence of molecular sensing materials (CoPc) on the response of the sensor in the presence of ammonia. All the measurement were carried out at room temperature.