PEG-8 dimethicone is a polyethylene glycol derivative of dimethicone. The structure of dimethicone consists of organic (carbon-based) groups attached to a linear inorganic backbone of silicon and oxygen. Based on this structure, dimethicone is considered to be a hydrophobic oil. One way to make dimethicone more water-soluble is by adding multiple units of ethylene glycol to sites along the polymer chain. The oxygen atoms add polarity to the silicone and are readily available for association with water molecules. This process is called ethoxylation or polyethylene glycol (PEG)-modification. The number associated with the ingredient name (i.e., PEG-8 dimethicone) corresponds to the number of repeat units of ethylene glycol.

PEG-8 dimethicone functions to condition, lubricate, and protect the skin. The combination of silicone with methyl groups tends to make it extremely resistant to water, yet it keeps them flexible and moving free, ideal properties for a lubricant. Due to its large molecular size, PEG-8 dimethicone is poorly absorbed into the skin. Therefore, upon application, PEG-8 dimethicone not only prevents water loss, but also protects the skin from penetration of harmful substances, such as irritants and bacteria.