In recent years, 1-butyl-3-methylimidazolium carboxylates ionic liquids (ILs) have been reported to be powerful solvents for the dissolution of cellulose and chitosan. However, very few studies have been conducted to investigate their basic physicochemical properties such as viscosity and conductivity. In this work, we have synthesized 1-butyl-3-methylimidazolium carboxylates ILs: 1-butyl-3-methylimidazolium formate ([C(4)mim][HCOO]), 1-butyl-3-methylimidazolium acetate ([C(4)mim][CH3COO]), 1-butyl-3-methylimidazolium propionate ([C(4)mim][CH3CH2COO]), and 1-butyl-3-methylimidazolium butyrate ([C(4)mim][CH3(CH2)(2)COO]), in which the alkyl chain length in the anions is varied. The experimental viscosities (eta) and conductivities (sigma) for these as have been determined at different temperatures, and well correlated by the Vogel-Fulcher-Tammann (VFT) equation. Walden plots indicate an increase in ion pairs and/or ionic aggregates in the ILs with increasing alkyl chain length. Furthermore, the effect of alkyl chain length in carboxylate anions on viscosities and conductivities of the ILs has been discussed in light of the contribution from van der Waals interactions. Such knowledge would be useful for the understanding of the structure-property relation:hip of ILs and for the rational design of novel task-specific ILs.