SiO2 was modified with H2N(CH2)(3)Si(OEt)(3) (mass ratio of 1.1:3) and then used to adsorption of rosmarinic acid from i-PrOH solns. (concns. 250-7200 mg/L) at 25 degrees C to det. Adsorption isotherms by using Redlich-Peterson, Dubinin-Astakhov and Langmuir models. The isotherm parameters were estd. from linear regression and nonlinear fitting anal. The equil. adsorption data were best fitted by isotherm models when non-linear regressions were used. Max. adsorption capacity of modified silica calcd. from the Langmuir isotherm reached 205.9 mg/g.