Flux Response Technology is a technique based on a capillary viscometer for the measurements of flow and viscosity changes. In previous works, it has been used to measure logarithmic viscosity gradients of ideal mixtures. Here, we extend its range of application to the measurement of non-ideal mixing volumes at low temperatures. The theory describing a Flux Response apparatus has been reformulated without assuming gas ideality and taking into account the mixing volume. A systematic error in the measurement of the viscosity has been identified and estimated. We report the excess volume measurements of CO2 + CCl2 F2 mixtures obtained at 273.15 K and 1.11 bar.