Extraction of Ca(II) picrate (CaPic2) and SrPic2 with 18-crown-6 ether (18C6) and benzo-18C6 (B18C6) into 9 diluents were studied at T = 298 K. Their ion-pair extraction constants (Kex,ip = [MLPic2]org/[ML2+][Pic-]2) were determined at M = Ca, Sr, L = 18C6, and B18C6, where the subscript “org” refers to an org or a diluent phase. The plot of log Kex,ip vs. log KD,L based on the regular solution theory (RST) yielded a molar volume (cm3/mol) ratio (V/VL) of MLPic2 against L as a slope and then the V values were evaluated. Here, KD,L = [L]org/[L] and its literature values and VL were employed. Also, apparent radii (r /Å) of the ion-pair complex, Pic-ML-Pic, in the phases were evaluated basically from the V values and the relation of V = 4?r 3/3. Moreover, the log ?2 values were obtained from intercepts of the RST plots only at the hydrophobic B18C6 systems: ?2 =[M(B18C6)Pic2]/[MB18C62+][Pic-]2. Additionally, the KD,MLPic2 (= [MLPic2]org/[MLPic2]) values for the 9 diluent/water systems were determined from the relation of KD,MLPic2 = Kex,ip/?2. Using these values, the Ca(II) and Sr(II) extraction with L and forms for their MLPic2 complexes were characterized, compared with the CdPic2 and PbPic2 extraction with 18C6 and/or B18C6.