The measurement of volume fractions and their spatial distribution in multiphase flows is of great interest in many industrial applications. Ultrasound methods are well-suited for the differentiation between liquid and gaseous phases. In a system for ultrasound process tomography, the signal transmission characteristics strongly vary with the composition and the spatial distribution of the phases in the multiphase flow. In this contribution, a simulation technique for ultrasound process tomography of gas / liquid flows is presented, which uses ray-tracing for the simulation of ultrasound wave propagation. The received signals obtained after transmission through the liquid phase and scattering at the interfaces between the liquid and the gaseous inhomogenities are efficiently calculated in the frequency domain. The simulation concept and some results of simulated exemplary scenarios are discussed.