In this article, measurements of glass transition temperature (T-g) changes of two energetic material blend systems were carried out using the differential scanning calorimetry (DSC) technique. On one hand, experimental T-g values were compared to those predicted by the additivity model, Fox and Pochan equations, and, on other hand, to atomistic molecular dynamics simulation results performed in this work. The two blend systems studied were both composed of a polymer, either the inert hydroxyl-terminated polybutadiene (HTPB) or the energetic polyglycidylazide (GAP), and smaller molecules, which acted as plasticizers, dioctyl adipate (DOA), or glycidylazide oligomers (Gp1). Modeling results show deviations from experimental data, which varied from 5 to 20 K over an absolute scale for pure components and blends. A good fit was found when predicting the effect of adding smaller molecules to HTPB. Simulations were particularly useful for the blend in which the glass transition temperature of one component, DOA, was not experimentally measurable, due to the high crystallinity of the small DOA molecule.