A comparative investigation of creep performance in an experimental single crystal base alloy (designated RR2086) and modified (with additions of carbon, boron and hafnium) RR2086 superalloys has been undertaken. The alloys were creep tested over a temperature range of 850-1050degreesC. At 850degreesC/430 MPa, the creep behaviour of the modified alloy was slightly better than the base material, particularly in the tertiary creep stage. But at both 950degreesC/210 MPa and 1050degreesC/165 MPa, the creep strain of the modified alloy evolved more rapidly, resulting in a shorter creep life than the base RR2086. Microstructure investigation showed that the interdendritic regions of modified RR2086 contained relatively large, irregular gamma' particles which rafted to a limited level during creep, and that the amount of pores was significantly reduced in the modified alloy owing to MC carbide formation. Analysis of the correlation between microstructure and creep performance revealed that the reduction of pores in the modified alloy was beneficial to creep behaviour at the tertiary stage, and resulted in a longer creep life in the modified alloy than in the base RR2086 at 850degreesC/430 MPa. However, large, irregular and partially rafted gamma' particles and brittle MC phase in the interdendritic regions of the modified RR2086 caused a higher creep rate during the primary and secondary creep stages. The detrimental effect increased with creep temperature, counteracting the beneficial effect of porosity reduction. As a result the modified alloy had a shorter creep life than the base alloy at 950 and 1050degreesC. (C) 2004 Kluwer Academic Publishers.