High temperature hydrogen attack (HTHA) has long been a concern for the petrochemical industry, as failure due to HTHA has had devastating consequences, including loss of human life. Accordingly, the American Petroleum Institute (API) established recommended practice guidelines for “safe” operating conditions for pressure vessels, based upon prior failures and experiences in-service, but without the benefit of vast experimental data. Consequently, with this API-funded project, we generated creep data for pressure vessel steel in hydrogen, at various temperatures, hydrogen pressures, and stresses to better understand HTHA mechanisms. We found that under some conditions, the tertiary creep rate plateaued, which was unexpected and suggests current models may under predict some lifetimes. Using microscopy to show the evolution of microstructures and creep data, we hope to elucidate HTHA mechanisms, improve predictions of creep behavior, and ultimately enhance recommended practices of operation for petrochemical equipment and equipment associated with the future hydrogen economy.