As an aircraft gains altitude, the temperature drops and the fuel's capacity to hold water is diminished.
Chemically, FSII is an almost pure (99.9%) ethylene glycol monomethyl ether (EGMME, 2-methoxy ethanol, APISOLVE 76, CAS number 109-86-4); or since 1994, diethylene glycol monomethyl ether (DEGMME, 2-(2-methoxy ethoxy) ethanol, APITOL 120, methyl carbitol, CAS number 111-77-3).
[1] Prior to 1994, Prist was regulated under the MIL-I-27686E standard, which specified use of EGMME, but subsequently came under the MIL-DTL-85470B, with use of less hazardous DEGMME with higher flash point.
[2] FSII was thought to retard the growth of microorganisms eventually present in the fuel, mostly Cladosporium resinae fungi and Pseudomonas aeruginosa bacteria, known as "hydrocarbon utilizing microorganisms" or "HUM bugs", which live in the water-fuel interface of the water droplets, form dark, gel-like mats, and cause microbial corrosion to plastic and rubber parts, but this has since been removed from labelling.
Drums containing FSII must be kept clean and dry, since the additive is hygroscopic and can absorb water directly from moisture in the air.