Tremendous attention has been devoted to understanding the behavior of common engineering alloys in high temperature hydrogen service. Nelson curves from API RP 9411 are available for the alloys most widely used in HTHA service. However, the aging international fleet of equipment in hydrogen service, across the refining, petrochemical and fertilizer industries means that in-service equipment constructed from less-common alloys must sometimes be evaluated for HTHA. The authors are involved with an 10-year joint industry project studying HTHA. One major outcome of this project is a volumetric HTHA damage FFS model which incorporates stresses from methane pressure within the material into an MPC Omega Multiaxial damage formula. A crack growth model using the fracture mechanics parameter C* (commonly used in creep crack growth)12 is also under development. These two are related in that the C* crack growth law parameters can be calculated utilizing the MPC Omega material creep properties. With sufficient creep data for an uncommon alloy like C-0-.3Mo, lot centered MPC Omega creep properties can be regressed. Such data have been compiled by the authors and resulting Omega creep properties are reported. These creep properties, along with other material properties, are used in the HTHA damage progression and the C* crack growth models. Model predictions using this approach are compared to historically available ex-service HTHA data from API RP 9411 for alloys with Mo contents between that of carbon steel and C-0.5Mo. Selective testing (creep and crack growth testing in both air and hydrogen environments) is in-progress to verify the FFS approach.