The tensile behaviour of different geothermal well cement formulations was investigated in order to determine how these materials perform under such loading and how tensile capacity can be improved. The influence of latex, perlite and fibres on the load-displacement relationship of the cements was measured on notched cylinders. The fracture surfaces were examined to further elucidate failure mechanisms. Unreinforced cements exhibited linear elastic behaviour to different degrees and failed in a brittle manner. Cements reinforced with either carbon (150 mum) or steel (1-3 mm) microfibres required greater loads for failure. However, the microfibres did not provide any major improvement in ductility. Addition of 13 mm steel fibres to the cements resulted in both strengthening and transition to ductile behaviour. Inclusion of the types of fibres studied in this work in cements offers potential benefits in maintaining the integrity of geothermal wells when tensile stresses are involved. (C) 2004 Kluwer Academic Publishers.