Theoretical oscillator strengths for 3s 2 3p 2 P-3s 2 3d 2 D, 3s 2 3p 2 P-3s 2 4s 2 S and 3s 2 4s 2 S-3s 2 4p 2 P fine-structure transitions in some ions of the aluminium isoelectronic sequence are reported. The computations have been carried out with two formalisms within the context of quantum defect theory: the quantum defect orbital method (QDO) and its relativistic counterpart (RQDO). The advantages of including relativistic effects are made apparent. Some of these are reflected by the correct systematic trends displayed by the RQDO f-values along the isoelectronic sequence.