The spelling of words in English is governed in part by the morphemes that make them up. This study examines the strength of children's knowledge of the role of root morphemes in spelling, specifically focusing on whether it can withstand interference by phonological changes. A total of 75 children between seven and nine years of age were given the first parts of two-morpheme transparent and opaque derived words (e.g. musical and objection) and one-morpheme control words (e.g. metal and portion). Children used the clues to a significant extent with the two-, but not the one-morpheme words. This effect was consistent across age groups and across both types of derived words. This is evidence that children's understanding of the link between morphemes and spelling is not impeded by changes in the sound of morphemes. These findings have substantial implications for models of spelling development and for educational practice. [This work was supported by a Rhodes Scholarship awarded to the first author.]