Congruency effects were examined using a manual response version of the Stroop task in which the relationship between the colour word and its hue on incongruent trials was either kept constant or varied randomly across different pairings within the stimulus set. Congruency effects were increased in the condition where the incongruent hue-word relations were maintained. This effect was observed even when only a single letter was coloured rather than the whole word. The data suggest that the Stroop interference effect can increase when words and hues become bound by learning, which can make it more difficult to ignore the word when responding to the hue. The implications for understanding Stroop interference are discussed.