For many molecules the determination of molecular quantities, as electric dipole moments or molar absorption coefficients, has to be based on measurements on solutions or other dense phases. The evaluation of such data is rather involved due to the interactions between neighboring molecules, and taking this into account, a general method is presented. Some quantities appropriate for the description of a macroscopic system (bulk quantities) are defined and their properties are discussed, especially partial molar quantities (PMQ's),which are well known from thermodynamics. To any PMQ some corresponding model molar quantities (MMQ's) can be introduced, each MMQ based on a particular molecular model. The MMQ's are related to the interesting molecular quantities and may allow their estimation. The values of PMQ's can be determined uniquely from experimental data, if the investigated system meets just a very few general requirements, whereas the values of the corresponding MMQ's are generally dependent on the chosen model and hence unique only, if a true model is available. As examples, approximate molecular models are introduced for MMQ's corresponding to the Gibbs energy and to quantities convenient for the evaluation of permittivity, optical absorption and electro-optical absorption measurements.