Padding gas or pad gas fills the space left in a tank when a liquid or heavier non-miscible gas is taken out.

The release of volatile organic compounds (VOCs), especially hydrocarbons, into the atmosphere during loading of marine vessel storage tanks long plagued the petroleum industry. Air or inert gases such as carbon dioxide or nitrogen were generally added to marine vessels and tanks upon unloading to fill the space created by pumping out liquid cargo. These gases are referred to as pad gases. Later, as liquid cargo was loaded, the pad gases were displaced by the liquid and had to be vented. The pad gases and any contained VOCs were then lost.
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