A 1 mM solution of crystal violet 1 in chloroform was found to be an intense purple colour (UV-Vis λmax = 589 nm). Addition of equimolar Me2S•BH3 complex resulted in the rapid loss of colour and formation of a colourless solution (UV-Vis, no absorption), ca. 60 seconds for complete loss of colour (Figure ). A control reaction of the addition of SMe2 alone to a chloroform solution of crystal violet (1 mM) resulted in no colour change, confirming the colour change observed was due to the BH3 added. Crystal violet 1 therefore has the potential to be a colour indicator for hidden borane catalysis.