Morphology is said to play a role in word acquisition. English psych adjectives are identified as a category of words with morphological complex gradience, which consists of free morphs, derivatives and inflections. Due to their morphological features, psych adjectives were considered the ideal candidates to test the effect of morphology on word acquisition. This study adopted a within-group research design, where a controlled productive test and a receptive knowledge test were developed and administered to English learners of an intermediate level. Results suggested that acquisition of the inflections was significantly different from that of free morphs and derivatives in the controlled productive test, but no difference was found in the receptive knowledge test. Comparisons of the two tests for the three word groups showed that the only significant difference occurred in the inflection word group. The overall results revealed that learners experienced much more difficulty in acquiring inflectional psych adjectives. In light of the dissociation of associative memory and rule memory, two possible explanations for the learning problems were offered： 1） learners failed to represent the inflections in associative memory, or 2） they have not proceduralized the morphology rules in rule memory.
