The plurals-inside-compounds effect has been one of the most widely studied morphological phenomena in the psycholinguistics literature. Research has shown that there is both a restriction against regular but not irregular plurals inside compounds (*rats eater versus mice eater), and a general preference for non-heads to be singular (?mice eater versus mouse eater). The general preference for singular non-heads has been derived from a semantic constraint against non-heads marked for plural number semantics (Haskell, MacDonald & Seidenberg 2003),whereas the avoidance of regular plurals results from a morphological constraint against concatenative regular plural inflections (Gordon 1985, Kiparsky 1982, Pinker 1999). However, a restricted range of exceptions to the morphological constraint (buildings inspector) are in fact possible, but only when licensed by particular semantic properties (Alegre & Gordon 1999). Whether these constraints also apply to word formation by derivational suffixation is not known (*ratsless versus ?miceless versus ~ouseless), and the time-course of these constraints during online processing has not previously been examined.