Intrinsic stress and microstructure of mixed composition films were investigated for several material systems useful in the IR wavelength range. Stress in coevaporated mixed films (analog) deviated from a linear dependence on composition, while in discrete alternating films (digital) it varied linearly with composition. For MgF2-Ge and CeF3-Ge, analog film stress was small and compressive over a large range of intermediate compositions despite the fact that the constituent materials had high tensile stress. The stress behavior correlated with changes in the microstructure. Low stress compositions were observed to correspond to enhanced grain size in the mixed film. These results have significant implications for the design of gradient-index thin-film structures.