Quantitative theoretical predictions of the carrier lifetimes in a number of imperfect GaxIn1-xSb-InAs superlattices are presented. Strain-dependent empirical pseudopotentials are used to provide a microscopic description of the stationary states in the structures and scattering theory is employed to extract lifetime information. The effect of interface islands is examined,. and lifetimes are found to depend upon the detailed size, shape, and composition of the islands. The effect of higher order multiple scattering events is seen to be significant. For isolated isovalent Sb substitutional defects in the InAs layers, a lifetime of approximate to 0.4 mu s is found to be typical. This is shown to be an order of magnitude shorter than in the case of As defects in the alloy layers.