Boron is used as a grain boundary strengthener in the nickel-based superalloys, but the reasons for its effect and the optimum quantities which need to be added are not well understood. Recently, some of the authors have developed a new corrosion-resistant single crystal superalloy for power generation applications which has a good balance of mechanical properties and resistance to environmental degradation. Here, this same alloy system is studied but in the polycrystalline state with additions of boron and carbon. The influence of boron on creep behaviour has been quantified and rationalised, with an emphasis on the grain boundary phase transformations which occur. NanoSIMS is utilised to investigate the segregation of boron at grain boundaries, with SEM used to characterise the effect of boron on the precipitation of Cr23C6 type carbides at grain boundaries. When the boron content increases, the agglomeration of M23C6 carbides at grain boundaries is inhibited. Uniformly distributed discrete M23C6 carbides are observed at higher boron content, whereas a deleterious film of M23C6 along the grain boundaries arises as the boron concentration is reduced. Boron promotes also the formation of γ′ layers at the grain boundaries.