The transient flow of gas in pipes can be adequately described by a one-dimensional approach. Basic equations describing the transient flow of gas in pipes are derived from an equation of motion (or momentum), an equation of continuity, equation of energy and state equation. In much of the literature, either an isothermal or an adiabatic approach is adopted. For the case of slow transients caused by fluctuations in demand, it is assumed that the gas in the pipe has sufficient time to reach thermal equilibrium with its constant-temperature surroundings. Similarly, when rapid transients were under consideration, it was assumed that the pressure changes occurred instantaneously, allowing no time for heat transfer to take place between the gas in the pipe and the surroundings. For many dynamic gas applications, this assumption of a process having a constant temperature or is adiabatic is not valid. In this case, the temperature of the gas is a function of distance and is calculated using a mathematical model, which includes the energy equation. In the paper, a comparison of different (isothermal and non-isothermal) models is presented. Practical examples have been used to emphasize differences between models. (C) 2001 Elsevier Science B.V. All rights reserved.