The paper presents a thermodynamic analysis of the removal of an inert gas from the tank using the vapor of liquefied petroleum gas cargo (called cargo tank gassing-up operation). For this purpose a thermodynamic model was created which considers two extreme cases of this process. The first is ‘piston pushing’ of inert gas using liquefied petroleum gas vapour. The second case is the complete mixing of both gases and removal the mixture from the tank to the atmosphere until desired concentration or amount of liquefied petroleum gas cargo in the tank is reached. On the example of nitrogen as inert gas and ethylene as a cargo, by thermodynamic analysis an attempt was made to determine the technical parameters of the process, i.e., pressure in the tank, temperature, time at which the operation would be carried out in an optimal way, minimizing the loss of cargo used for gassingup. Calculations made it possible to determine the amount of ethylene used to complete the operation and its loss incurred as a result of total mixing of both gases.