Lipophilicity, usually expressed as octanol-water partition coefficient (logP o/w), is an important property in biomedical research, drug design and technology. However, high logP o/w values of complex hydrogen-bonding molecules are not easy to measure or calculate. Exemplary problematic molecules are prospective active components (ionophores) of polymeric sensor membranes – the working elements of ion-selective electrodes. High lipophilicities of the membrane components are crucial for the sensor lifetime. In this work, lipophilicities of a wide range of urea-, carbazole- and indolocarbazole-based anion receptor molecules (some newly synthesized) and two common plasticizers were determined using a chromatography-based approach and/or the COSMO-RS method. Very high logP o/w values, up to around 20, i.e. far beyond directly experimentally accessible range, were obtained. The agreement between the two approaches ranged from very good to satisfactory. Based on these results, simple fragment-based equations were developed for quick lipophilicity estimation without any specialized software. Membrane-water partition coefficients for the studied compounds were modeled. Limitations and biases of the used methods are discussed.