The economic due diligence that determines the price to pay for a gas property can be sensitive to the time value of money. This calculation is therefore intimately linked to predicting flow rates. With the invention of highly sensitive technology, gas-phase viscosities can now be measured accurately. This enables the investigation of previously ignored effects such as the influence of equilibrium water vapour on gas-well deliverability. This paper discusses the technology of gas-phase viscosity measurement and then compares existing literature data. The paper then presents, by means of a case history, the influence of water from condensation on viscosity and concomitantly on production rates. Moreover, material-balance calculations are provided that show the role of water vapour in gas properties evaluation. It is concluded that calculated gas flow rates can be significantly impacted by incorporating the influence of equilibrium water vapour. whereas the influence of water vapour on gas in place is minimal.