We describe our current efforts employing a combination of electron microscopy and synchrotron-based characterization techniques to quantify the effects of extended thermal ageing (up to 50,000hr at 360-700°C) on the microstructural properties of HT9, T91 and T92 ferritic- martensitic alloys. Changes in the microstructure were readily apparent from the qualitative and quantitative XRD analysis. Specific phases observed include BCC Fe host, FCC M23C6, HCP Laves (FeMo2), and FCC MX phase, with microstructures (weight fractions, lattice parameters and coherent grain sizes) all quantified as a function of ageing conditions. When coupled with electron microscopy, synchrotron-based XRD provides complimentary, and high throughput, insight across the multiple length scales needed to fill critical knowledge gaps to predict long-term behavior and performance in these important alloys. We highlight opportunities in leveraging synchrotron-based techniques to address applied materials science challenges to aid in developing physical understanding of thermally-induced microstructures in materials for future energy applications.