The acidity function of boron trifluoride-phosphoric acid-water, BF3.xH(3)PO(4).yH(2)O (3), was determined by the C-13 NMR Spectroscopic method from the Delta delta(0) parameter, using mesityl oxide as an indicator. As expected, the increase in concentration of BF3 increases the acid strength. The change in acidity with the total amount of hydroxylic components (x + y) between 1 and 4.5 is less steep for 3 than for the corresponding hydrates, BF3.(x+y)H2O (4). For x + y < 1.5, the hydrates (4) are stronger than the ternary mixtures (3). For x + y > 1.8, the ternary mixtures are stronger. Phosphoric acid in the mixture forms polyphosphoric acid and liberates water, which complexes with BF3. The degree of polycondensation varies with the ratio of water to BF3 in 3. Phosphoric acid has, therefore, a buffering effect on the acid strength.