The nonlinear refractive index n2 of SiO2-Al2O3-La2O3 (SAL) glasses of 10 to 24 mol% La2O3 is determined via Z-scan technique at 800 nm in the sub-100 fs time regime. n2 (5.8 to 9.3 × 10−16 cm2/W) correlates linearly with the La2O3 concentration scaling by the factor (0.264 ± 0.007)×10−16 cm2/W per mol% of La2O3. The relation between n2 and the linear refractive index n0 of the SAL systems is successfully described by the theoretical model of Boling, Glass and Owyoung (BGO theory). Increasing concentrations of heavy La3+ ions and non-bridging oxygen accompanied by a rising volume of the O2− ions are considered to be responsible for the n0 and n2 increase as well as a slight Urbach energy decrease. The upper limit of the La2/3O hyperpolarizability is estimated to amount to (2.2 ± 0.2)×10−36 esu.