Jet reactors are characterized by their compactness, high shear and high interfacial areas per unit volume. Atomization of the liquid feed and/or turbulent gas-liquid mass transfer forms the basis of the jet reactor technology. A mathematical model of visbreaking of Athabasca bitumen in such a reactor is presented. The model is based on single phase (bitumen) flow and uses the well known two equation turbulence model. Model predictions are compared with limited experimental data and they agree well.