High strength steel fasteners characterised by tensile strengths above 1100 MPa are often used in critical applications, where a failure can have catastrophic consequences. Preventing Hydrogen Embrittlement (HE) failure is a fundamental concern implicating the entire fastener supply chain. In this work, testing and observations made on nine alloys, each quenched and tempered to four hardness levels, illustrate second order differences in susceptibility that are dependent on chemistry and heat treatment, and ultimately on microstructure.