The partitioning of an organic solute between an aqueous and an organic phase is governed by the properties of the phase forming aqueous/organic system as well as by the properties of the solute. As organic solutes are often highly diluted in such systems the infinite dilution partition coefficient is of particular interest. Partition coefficients at infinite dilution are often described using quantitative structure-activity relationships (QSAR), e.g. the linear solvation energy relationship (LSER) method of Kamlet and coworkers. The LSER method requires five parameters for the characterization of a solute j: a volume parameter nu (j), a polarity parameter pi (j), a parameter for the polarizability delta (j), and two parameters describing the capability of the solute to undergo hydrogen bonding as a proton acceptor beta (j) (basicity parameter) and as a proton donor alpha (j) (acidity parameter). This contribution reports new experimental data for the polarity parameter pi (j) and the basicity parameter beta (j) of about 90 liquid organic compounds. Such parameters can be used to predict thermodynamic properties such as the octanol-water partition coefficient or to develop methods for estimating such parameters.