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If recognition of a polymorphemic word always takes place via its decomposition into stem and affix, then the higher the frequency of its stem (i.e., base frequency) the easier the lexical decision response should be when frequency of the word itself (i.e., surface frequency) is controlled. Past experiments have demonstrated such a base frequency effect, but not under all circumstances. Thus, a dual pathway notion has become dominant as an account of morphological processing whereby both decomposition and whole-word access is possible. Two experiments are reported here that demonstrate how an obligatory decomposition account can handle the absence of base frequency effects. In particular, it is shown that the later stage of recombining the stem and affix is harder for high base frequency words than for lower base frequency words when matched on surface frequency, and that this can counterbalance the advantage of easier access to the higher frequency stem. When the combination stage is crucial for discriminating the word items from the nonword items, a reverse base frequency effect emerges, revealing the disadvantage at this stage for high base frequency words. Such an effect is hard for the dual-pathway account to explain, but follows naturally from the idea of obligatory decomposition.