The influence of morphological knowledge on lexical processing and acquisition: The case of Arab EFL learners Ahmed M. Masrai Department of English Language and Literature, Swansea University, Swansea, Wales SA2 8PP, UK highlights  Morphological knowledge links to L2 vocabulary acquisition.  Regular inflectional morphology explains approximately 38% of the variance in L2 Vocabulary uptake.  Derivational morphology does not explain L2 vocabulary development.  L1 Arabic morphological rules do not contribute to L2 morphological awareness. article info Article history: Received 11 November 2015 Received in revised form 23 February 2016 Accepted 11 April 2016 Available online 14 April 2016 Keywords: Morphological processing Vocabulary acquisition Arabic Inflection Derivation abstract Although morphological knowledge has been proposed to enhance second language (L2) vocabulary acquisition, little is known about which morphological process has the greatest impact on lexical acquisition. To address this question, 400 school-learners of English from high schools in Saudi Arabia were presented with a morphological decomposition task of regular and irregular inflection and deri- vation, and an L2 vocabulary size test. The results indicated some significant levels of correlation be- tween knowledge of regular inflection and derivation, and L2 vocabulary knowledge. Irregular inflection and derivation, on the contrary, were not found to have a significant effect on L2 vocabulary acquisition. Although significant correlations were observed between regular morphology and L2 vocabulary learning, regression analysis showed that only regular inflection processing has a sizable effect on vo- cabulary uptake. This variable explained about 38% of the variance per se. The findings also revealed no clear effect of the first language (L1) regularity of morphological rules, which apply extensively in Arabic, on acquiring words that are regular in English. The overall findings propose an explicit focus on teaching regular inflectional morphology in the language classroom because of its marked influence found on vocabulary acquisition. © 2016 The Author. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1. Introduction Understanding how words are formed is potentially a key component in developing a sizeable second language (L2) lexicon. Additionally, as vocabulary knowledge increases L2 learners should gain insights into morphological processing of the target language. Morphological knowledge has been studied extensively on the literature of L2 acquisition (e.g. Refs. [14,15,19,20,36,44]), and mastery of morphological structure has long been proposed to be linked to vocabulary acquisition [8,41]. However, there is relatively scant research that has attempted to explore the link between L2 learners' ability to manipulate the morphological elements of words and the development of their vocabulary size with native Arabic speakers. This study, therefore, is an endeavour to explore this assumption among native Arabic learners of English as a foreign language (EFL) in Saudi Arabia, where no study, to the au- thor's knowledge, has been conducted. Studies of vocabulary acquisition in Saudi Arabia repeatedly show a small EFL vocabulary gain by schoolchildren (e.g. Refs. [2,3,5,32]). Potential factors underlying this poor vocabulary uptake are not adequately investigated. There are two studies found in the literature that have endeavoured to explore this E-mail address: a.m.masrai@swansea.ac.uk. Contents lists available at ScienceDirect Ampersand journal homepage: www.elsevier.com/locate/amper http://dx.doi.org/10.1016/j.amper.2016.04.001 2215-0390/© 2016 The Author. Published byElsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Ampersand 3 (2016) 52e60