For use in sensitivity studies and robust optimization, uncertainties in thermodynamic models have to be considered in simulations. Instead of using less vivid confidence intervals of the models' fitting parameters, this work proposes a perturbation scheme for the fugacity coefficients in the liquid phase which can be applied to any thermodynamic model including equations of state. The perturbation is tuned via physically vivid and comprehensible parameters which have a direct relation to experimental uncertainties. Perturbations of pure component properties and mixture properties are separately accessible. After giving the theoretical framework of the perturbation scheme, the benefits of the approach are demonstrated using process simulation examples.