The present study is focused on determination of the liquid-liquid equilibrium (LLE) data for (water + phosphoric acid + 2-pentanol or 4-methyl-2-pentanol) ternary systems at T= (298.2, 308.2, and 318.2) K and p = 0.1 Mpa. Despite of the structural similarity of the chosen sec-alcohols, their polarity and hydrophobicity are different. Thus, the influence of the chemical structure as well as temperatures on the liquid phase behaviour was investigated. Solubility and tie line data were measured using different titration methods such as cloud point, acidimetric, and the Karl Fisher titration techniques. The refractive index measurements were also carried out on the aqueous phase. The experimental tie line data were correlated using the thermodynamic models of NRTL and UNIQUAC. The binary interaction parameters were optimized by the MATLAB fminsearch function. Both the ternary systems exhibit type-1 phase behavior. The quality of the tie-lines was checked using the Othmer-Tobias equation. Distribution coefficients and separation factors were calculated over the biphasic regions. (C) 2019 Elsevier Ltd.