Five propionic acid ionic liquids (PrAILs) [C(n)mim][Pro] (n = 2-6) (1-alkyl-3-methylimidazolium propionate) have been prepared by the neutralization method and characterized by H-1 NMR spectroscopy and differential scanning calorimetry (DSC). Their density, rho, surface tension, gamma, and refractive index, n(D), were measured at (298.15 +/- 0.05) K, and the experimental values of parachor for the PrAILs were calculated. Using the parachor values of [C(n)mim](+) obtained by Guan et al., the anionic parachor values of [C(n)mim][Pro] (n = 2-6), [C(2)mim][RBF3] (R = N-CnH2n+1 (n = 1 - 5)), [C(n)mim][Gly] (n = 2-6), and [C(n)mim][PF3(CF2CF3)(3)] (n = 1-6) were determined. Then, the parachor, surface tension, and refractive index of the ILs investigated in this work were estimated. The estimated values correlate quite well with the corresponding experimental values.