Bubble and drop coalescence phenomena observed in many industrial separation processes and in multiphase chemical reactors such as bubble columns and stirred vessels, often have a significant influence on the process performance. Even though a number of sophisticated modelling concepts have been presented in the literature over the years, the chemical and physical mechanisms involved are still not satisfactorily understood. Among the most promising methods applicable for elucidating these phenomena are the 'volume of fluid (VOF)' and the direct numerical simulation (DNS) methods. On the other hand, the multifluid models have been found to represent a trade-off between accuracy and computational efforts for practical applications. In these multifluid models constitutive equations are needed to describe the coalescing process, and due to the limited understanding of these phenomena we still have to resort to empirical correlations. The present model belongs to the latter class and deals with the collision process between two fluid particles. The intention is to formulate a constitutive model that can be applied for determining whether the particles coalesce or not. The model should be applicable for both phenomenological reactor models and CFD-based reactor models. Turbulence is included by using collision time scales generally less than a second, and by including fluid particle shape oscillations.