The working mechanism of poly(vinyl alcohol) (PVA, M-w similar to 200,000 g mol(-1)), a fluid loss control additive (FLA) applied in oil well cementing, was investigated. First, characteristic properties of PVA such as solubility and particle size in cold and hot water, minimum film forming temperature, adsorption on cement, viscosity of cement pore solution and static filtration properties of cement slurries treated with PVA were determined. It was found that the working mechanism of PVA relies on hydrated, but water-insoluble PVA particles (d(50) similar to 2.4 mu m). During cement slurry filtration, they coalesce into a polymer film. This film effectively plugs the pores of the cement filter cake. The sample studied here becomes water-soluble at temperatures > 40 degrees C (d(50) decreases to similar to 50 nm) and looses its effectiveness. Addition of highly anionic dispersants such as beta-naphthalenesulfonate formaldehyde (BNS) or acetone formaldehyde sulfite (AFS) poly-condensate extends the temperature range at which PVA works from 40 degrees C to similar to 60 degrees C. This effect is ascribed to lower solubility of PVA in the presence of these dispersants. The study reveals that decreased performance of PVA caused by higher temperatures is not the result of thermal degradation of the polymer, but is owed to its increasing water-solubility. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 2290-2298, 2010