Carboxylate-anion-based imidazolium ionic liquids (ILs) are powerful solvents for cellulose and lignin. However, little is known about their fundamental physicochemical properties. In this work, 1-butyl-3-methylimidazolium carboxylate ILs 1-butyl-3-methylimidazolium formate ([C(4)mim][HCOO]), acetate ([C(4)mim][CH3COO]), propionate ([C(4)mim][CH3CH2COO]), and butyrate ([C(4)mim][CH3(CH2)(2)COO]), in which the alkyl chain length in the anions is being varied in contrast to the more usual studies where alkyl chain length in the cations is varied, have been synthesized and their densities and surface tensions have been determined experimentally at different temperatures. By using these data, the molar volume, isobaric expansivity, standard entropy, lattice energy, surface excess entropy, vaporization enthalpy, and Hildebrand solubility parameter have been estimated for these ILs. From the analysis of structure property relationship, the effect of alkyl chain length in the anions on these physicochemical properties of the ILs has been assessed and the dissolution of cellulose and lignin in these ILs has been discussed. Such knowledge is expected to be useful for understanding the nature of this class of solvent for the dissolution of biomacromolecules.