Dynamic behavior of the flow field in a Reaction Injection Molding, RIM, machine mixing chamber, having dimensions typically used in industrial machines, is studied from dynamic velocity data of Laser Doppler Anemometry, LDA, measurements and Computational Fluid Dynamics, CFD, simulations with a 2D model. This stud-v is based on the spectral analysis of the dynamic flow, field data. The typical frequencies, in the reactor flow field, are identified and its values are related to the identified,flow structures. The differences between the typical frequencies from experiments and simulations are observed and justified on the basis of the 2D representation of a 3D cylindrical geometry. (C) 2009 American Institute of Chemical Engineers AIChE J. 55: 1338-1351, 2009