Infrared laserprobe 40Ar/39Ar dating has been used to date pseudotachylite and host-rock minerals from a crush belt in the Lewisian basement of Scotland. It has revealed complexity in the pseudotachylite data that is attributable to the presence of refractory host-rock clasts and mineral fragments in the pseudotachylite. In conjunction with the host-rock mineral laserprobe 40Ar/39Ar data it has been possible to simplify the pseudotachylite data for the samples, and the preferred ages for these are: 980 ± 39 Ma, 999 ± 31 Ma and 1024 ± 30 Ma (2σ). These ages are the first record of Grenville-aged brittle deformation in the Lewisian. Further, this study serves to illustrate the complexity of dating pseudotachylites, and the advantages and limitations of the IR laserprobe applied to such materials.