Spitzer IR Array Camera (IRAC) mid-infrared photometry is presented for the globular cluster (GC) systems of the NGC 5128 ('Centaurus A') and NGC 4594 ('Sombrero') galaxies. Existing optical photometric and spectroscopic are combined with this new data in a comprehensive optical-to-mid-IR colour catalogue of 260 GCs. Empirical colour-metallicity relationships are derived for all optical-to-mid-IR colour combinations.
