The effect of hydrogen on the upper bainite transformation in two silicon containing steels has been investigated. For comparison, isothermal transformation at the same temperature has also been performed in a helium atmosphere. In both Fe-0.2C-3Mn-2Si and Fe-0.4C-4Ni-2Si (nominal wt %) alloys it was discovered that the bainite reaction proceeds further towards completion when the transformation is carried out in a hydrogen atmosphere. This can result in the reduction or elimination of the martensite phase which forms from residual austenite upon quenching to room temperature. The resultant microstructure of specimens heat treated in hydrogen was a fine aggregate of upper bainitic ferrite and interlath retained austenite. This effect is discussed in terms of hydrogen interactions in the lattice undergoing bainite transformation via a displacive mechanism. Additionally, it is found that the stability of the retained austenite in the final bainitic microstructure is not markedly influenced by hydrogen.