For the separation of species having very similar vapor pressures, adsorption is a promising separation method. The knowledge of adsorption isotherms as well as capillary condensation phenomena is necessary for process design and control. Usually, these properties were obtained by experiments. One technical important example is the purification of n-aldehydes produced via hydroformylation of olefins. In the case of n-aldehydes, no experimental information could be found in the literature. Therefore, a theoretical approach to calculate adsorption isotherms will be very helpful. Density functional theory (DFT), in which the thermodynamic properties are expressed as functionals of the spatially varying density, is applied for the theoretical modeling of adsorption isotherms of pure n-aldehydes and n-aldehyde mixtures. In this work, the free energy terms of the grand potential function are calculated from the Peng-Robinson EOS in order to model real molecules, for instances n-aldehydes and their mixtures. The interaction between the fluid and the wall is characterized by an external potential and in this work a Steele 10-4-3 potential is assumed. On this background calculations are provided for one dimensional slit pores over a wide pressure range, starting from the diluted gas until the condensate liquid phase. Effects of temperature change, wall potentials and n-aldehyde chain length are investigated. The obtained adsorption isotherms show that adsorption can be a promising technology for the separation of long-chain n-aldehydes having a very similar boiling point; however, further optimization is required. (C) 2016 Published by Elsevier B.V.