The moisture uptake of the three unsaturated fatty acids, oleic acid, linoleic acid, and linolenic acid has been investigated. The amount of water absorbed by the fatty acids increased with increasing degree of unsaturation. The influence of the water on the adsorption onto steel and lubricating properties of the fatty acid in hexadecane was investigated. The adsorption, as determined with quartz crystal microbalance of 0.1 and 1 wt% of oleic acid onto steel was not affected by the presence of water (1.5 mole% water in fatty acid) while 5.6 mole% water enhanced the adsorbed amount of linolenic acid. Even though this amount of water had a large influence on the adsorption of linolenic acid, it did not affect the friction and wear measured with high frequency reciprocating rig using oleic acid or linolenic acid in hexadecane.