Flavonoids are a large family of natural compounds that includes flavones, flavonols, and chalcones, among others. These compounds are investigated due to their varied biological properties and their capacity to complex metal ions. In this work, the complexation of four 3-hydroxyflavones with the lanthanum(Ill) ion was studied. A series of novel complexes of La(III) with the selected 3-hydroxyflavones, 3,7-dihydroxyflavone (L-1), 3,7,3'-trihydroxyflavone (L-2), 3,5,7,3',4'-pentahydroxyflavone (quercetin) (L-3), and 3,5,7,2',4'-pentahydroxyflavone (morin) (L-4), were studied by means of potentiometric and spectrophotometric methods at 26.3 degrees C and 0.1 M ionic strength in a water-dioxan (1: 1, v/v) medium. Their formation constants were evaluated by the Calvin-Bjerrum and Irving-Rossotti methods. Complex 1:1 was found for all four ligands, whereas the 2:1 ligand-metal stoichiometry was found for only three of the four ligands. The obtained protonation constants of the ligands were: L-1, log K-1(H) = 9.128 +/- 0.664; L-2, log K-1(H) = 11.091 +/- 0.134, log K-2(H) = 8.822 +/- 5 center dot 10(-4); L-3, log K-1(H) = 9.310 +/- 0.119, log K-2(H) = 8.382 +/- 0.109; L-4, log K-1(H) = 10.021 +/- 0.355, log K-2(H) = 7.173 +/- 0.576. The determined stability constants were: L-1-La(III), log K-1 = 7.269 +/- 0.387; L-2-La(III), log K-1 = 9.164 +/- 0.020, log K-2 = 5.200 +/- 0.185; L-3-La(III), log K-1 = 8.143 +/- 0.284, log K-2 = 5.632 +/- 0.585; L-4-La(III), log K-1 = 7.407 +/- 0.596, log K-2 = 4.830 +/- 0.165.