The mutual affinity of volatile organic compounds (VOCs) and the active polymer layer of dense organic solvent nanofiltration (OSN) membranes is an important property that influences the separation efficiency. Therefore, the affinity between active layer materials and VOCs was investigated by measuring the liquid liquid equilibria (LLE) between neat polymers used as active layer materials and VOCs. The measured phase equilibria are modeled using the thermodynamic PC-SAFT (perturbed chain statistical associating fluid theory) model. This model allows for estimation of the pressure- and temperature-dependent chemical potentials of VOCs and membrane materials in feeds, membranes, and permeates. Two polyimides (P84 and Matrimid 5218) were selected as typical OSN membrane materials, and five different VOCs (n-hexane, ethyl acetate, 2-propanol, ethanol, and toluene) were considered. It was found that all miscibility gaps between the polyimides and VOCs are open miscibility gaps. The miscibility gaps of Matrimid 5218 and VOCs are much smaller than those of P84 with all VOCs. By means of the PC-SAFT equation of state, the phase behavior of the two polyimides with all considered VOCs could be well described.