Herein a colourimetric indicator for hidden borane catalysis is reported (Scheme 1D). Crystal violet, (tris(4-(dimethylamino)phenyl)methylium chloride), was found to change from a purple solution to a colourless solution in the presence of BH3 , in solutions of toluene, chloroform and dichloromethane. This method has been shown to identify BH3 formed in situ from HBpin decomposition by species from across the periodic table by direct a 'catalyst' with HBpin, reaction sampling and even in situ.