In this paper a particle-particle interaction term is derived and incorporated into the two-fluid analytical model of Smit et al. (2011). This model was developed for low concentration spherical particle motion in a Newtonian fluid and the inclusion of particle interactions is required for instances where particles collide. Moreover, such a modification serves as a first step towards the modelling of higher particle concentrations. A brief overview of the analytical derivation of the model by Smit et al. (2(111) is included for clarity and a detailed derivation of the newly developed particle-particle interaction term is given. In this derivation, particle interaction is described using impulse mechanics with a collision sphere model in a centre of mass reference frame for collision detection. The updated model is included into an existing Fortran 95 program and validated with experimental data obtained by the authors from camera- and settling tube procedures. (C) 2013 Elsevier Ltd. All rights reserved.