Activity coefficients at infinite dilution, gamma(infinity)(i), of 13 solutes such as alkanes, alkenes, alkylbenzenes, alcohols, esters, and ethers in six 1,3-dialkylimidazolium- or tetraalkylphosphonium-based ionic liquids have been determined by gas chromatography using the ionic liquids as the stationary phase. Furthermore, the solubility of 5-hydroxymethylfurfural (HMF) in these solutes and the solubility of the solutes in 1-butyl-3-methylimidazolium methanesulfonate ([C(4)mim][CH3SO3]) was assessed. The combination of these data allowed for the interpretation of prevailing interactions on molecular level and resulted in the hypothesis that an ideal extracting agent must feature hydrogen bond acceptor properties to obtain high extraction efficiencies of hydrogen bond donor molecules such as HMF from this ionic liquid. Extraction data obtained proposed extracting agents demonstrated that this hypothesis was correct and can in future be transposed to other separation problems. In the case of a multifunctional molecule such as HMF, extraction efficiencies are however in general low and can only be little improved by removing other potential interaction sites from the ionic liquid's cation: hence, in the N-ethyl-N-methylpyrrolidinium analogue [C(2)C(1)pyr][CH3SO3], pi-pi-interactions between cation and HMF cannot form, which increases the extraction efficiency somewhat.