Poorly specified liquid mixtures, composed of specified components and unknown components, play an important role in many parts of the process industry. The unknown components may have a crucial impact on process design and performance, which is why it is necessary to consider the unknown part of poorly specified mixtures in thermodynamic models used for process design or optimization. This work introduces a perturbation scheme to model the impact of unknown components on the activities of the specified components within poorly specified mixtures. The prerequisite is a good thermodynamic model of the specified subsystem, which is then perturbed to consider the effect of the unknown part of the mixture. No detailed analysis of the unknown part of the mixture is needed to use the perturbation scheme. The novel perturbation scheme is applied to several example mixtures to illustrate the procedure of parametrization and simulation.