The VIC mixer is an intermeshing internal mixer, whose unique feature is its ability to vary the clearance between the two rotors. The flow field in the chamber of an internal mixer is difficult to analyze due to the complex geometry and transient character of the flow. Here, a fluid dynamics analysis package - FIDAP, using the finite element method was employed to simulate the flow patterns in a VIC 1.9 mixer. A 3D flow analysis was carried out for the whole mixing chamber. The problem of time dependent flow boundaries was solved by selecting a number of sequential geometries to represent a complete mixing cycle. The flow field was characterized in terms of velocity profiles, pressure contours, shear stresses generated and a parameter lambda quantifying the elongational flow components. The last two parameters are the most important ones in analyzing dispersive mixing efficiency. Distributive mixing was studied numerically by means of tracking the evolution of particles originally gathered as clusters. The pairwise correlation function and the particle concentration were used to quantify the degree of mixing. The dynamic average flow field characteristics based on the particle tracking were also calculated. The effect of changing gap size on dispersive and distributive mixing was investigated.