The structure and morphology in the meta-nitroaniline (MNA) dispersed poly(methyl methacrylate) (PMMA) guest/host system was investigated with respect to composition and different techniques of crystallization. Large changes in the intensities of various X-ray diffraction peaks were observed for both solution grown (SC) and melt crystallized (MC) MNA/PMMA composite films. However, in the former case, the peak corresponding to (060) reflection had maximum intensity, while in the latter, the peak corresponding to (112) reflection was most prominent. At low concentrations, the films appeared to be amorphous and transparent, but electron diffraction studies revealed small ordered domains. Both solution and melt grown MNA/PMMA films contained two types of crystallites having different structures: one having the original orthorhombic structure (a = 6.51 Å, b = 19.35 Å, c = 5.07 Å) of bulk MNA, while the other crystallized in new monoclinic phase with a = 13.0 Å, b = 19.35 Å, c = 10.15 Å, and β = 64°. The presence of the new structure that formed could be due to a formation of complex between PMMA and MNA. This complex formation and new structure was further confirmed by differential scanning calorimeter (DSC) and infrared analysis (IR). © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1547–1557, 2001