Understanding the film formation mechanisms, in particular for thermosetting polymeric systems, has attracted considerable interests of experimentalists 1-3 as well as theoreticians 4-7 in recent years. While laboratory measurements provide the opportunity to examine the effect of external and internal stimuli (temperature, molecular weight, reactivity of individual components, solubility, and others) on the film formation, 8-10 computer simulations offer other advantages in understanding the macroscopic properties evolving from the microscopic details in response to such stimuli. 11 For example, the effects of molecular weight, driving field, and temperature on the film formation can be studied effectively. 12,13 This is particularly important when multicomponent systems are considered; even though there are difficulties associated with computational analysis because of the increased number of parameters and chemical reactivity of the system, computer simulations remain the useful theoretic tool to probe such a complex system. However, when theoretical and experimental studies are carried out concurrently, a wealth of complementary information can be obtained regarding the highly complex multicomponent systems. One example of such complexity is represented by the two-component waterborne polyurethane (2K WB PUR) polymers, 14 which appear to be quite sensitive to numerous external and internal stimuli, such as relative humidity, miscibility of individual components, reactivity, evaporation rates, temperature, as well as rheological characteristics. In essence, in addition to chemical reactions, hydrophobic and polar interactions, which are not easily measurable, may play an important role. Further complications arise when such systems are constrained by the surface of the substrate on which the film formation occurs. Thus, a number of physical and chemical processes come into play simultaneously, which may be manifested by stratification, miscibility, or phase separation during the film formation, to name just a few.In this paper, we combined computer simulations 13,15,16 and compared them with the experimental findings from the recently developed internal-reflection IR imaging (IRIRI) 17 measurements in an effort to correlate the morphological evolution of the film formation in 2K WB PUR. Before we focus on these results, let us first identify the macroscopic morphological changes resulting from the changes of the cross-linking conditions. As shown in Figure 1, the optical images of the same PUR films cross-linked at different temperatures indicate significant morphological changes. Thus, the main questions are as follows: (1) What is the origin of the morphological features in addition to the chemical reactions leading to the formation of polyurethane/polyurea (PUR/PUA)? (2) What is the observed surface heterogeneity/roughness, the interface/ surface phenomenon, the reflection of the bulk behavior at the film surface, or both? To address these issues, polyurethane films were prepared using polyester resin disp...