The visual system successfully binds the shapes and colours of objects; therefore, our visual experience regarding the objects around us is coherent. However, this binding process can break down when attention is diverted, producing illusory conjunctions (ICs); for example, when presented with a red 2 and a green 5, the observer may report a green 2 and a red 5. The strongest observation of binding in human cognition is found in synaesthesia. In grapheme–colour synaesthesia, linguistic stimuli (e.g., letters or numbers) are strongly associated with colours. It is debatable whether these highly stable bindings constitute a form of early binding that occurs outside the focus of attention. We examined for the first time the occurrence of ICs in grapheme–colour synaesthesia. Experiment 1 replicated our previous finding, showing the effects of numerical distance on ICs (Arend et al., Psychon. Bull. Rev.2013, 20, 1181–1186). Participants viewed a display containing two centrally presented letters and two coloured numbers and were asked to report: (1) whether the letters were same/different, (2) the colour of the larger number, and (3) the level of confidence concerning the colour of the number. Experiment 2 used a modified version of this task. Synaesthetes () and controls () viewed number–colour pairs that were congruent or incongruent with that of the synaesthetic association. Grapheme–colour synaesthesia significantly affected ICs on incongruent but not on congruent trials. Our findings strongly support the notion that shape and colour are free-floating features in synaesthesia, similar to what is observed in normal cognition.