This paper reports the density, refraction index, and (vapor + liquid) equilibria (VLE) for binary systems {aldehyde + 1-ethyl-3-methylimidazolium ethylsulfate ([emim][EtSO4])}: {propionaldehyde + [emim][EtSO4]} and {valeraldehyde + [emim][EtSO4]}. The uncertainties for the temperature, pressure, and compositions measurements for the phase equilibria are +/- 0.1 K, +/- 0.01 kPa and +/- 0.0004, respectively. A qualitative analysis of the variation of the properties with changes in solvent and temperature was performed. The Peng-Robinson equation of state (PR EoS), coupled with the Wong-Sandler mixing rule (WS), is used to describe the experimental data. To calculate activity coefficients we used three different models: NRTL, UNIQUAC, and COSMO-SAC. Since the predictive liquid activity coefficient model COSMO-SAC is used in the Wong-Sandler mixing rule, the resulting thermodynamic model is a completely predictive one. The prediction results for the density and for the (vapor + liquid) equilibria have a deviation lower than 2.3% and 1.6%, respectively. The (vapor + liquid) equilibria predictions show a good description for the propionaldehyde system and only a qualitative description for the valeraldehyde system. (C) 2011 Elsevier Ltd. All rights reserved.