Language Learning ISSN 0023-8333 EMPIRICAL STUDY The Effects of Mode and Task Complexity on Second Language Production Olena Vasylets, a Roger Gilabert, a and Rosa M. Manch ´ on b a University of Barcelona and b University of Murcia Taking a psycholinguistic orientation within task-based language teaching scholarship, this study investigated the effects of mode (oral vs. written) and task complexity on second language (L2) performance. The participants were 78 Catalan/Spanish learners of English as a foreign language. Half of the participants performed the simple and complex versions of an argumentative, instruction-giving task orally, the other half did it in writing. The comparison of the participants’ oral and written performance revealed that speakers produced more idea units but that writers achieved higher scores for subordination, mean length of analysis-of-speech units, lexical diversity, extended idea units, and time on task. As for the effects of task complexity, the participants’ written production showed more variation between the complex and the simple versions of the task. These findings are interpreted in light of task modality effects in L2 learning and discussed in relation to task complexity theory and research. Keywords mode; speech; writing; second language; task complexity; propositional complexity The research reported in this article is part of the first author’s Ph.D. dissertation conducted under the supervision of the second and third authors and within the research activities of the language acquisition research group Grup de Recerca en Adquisici´ o de Lleng¨ ues based at the University of Barcelona. It is also part of a program of research led by the third author and funded by the Spanish Ministry of Economy and Competiveness (Research Grant FFI2012-35839) and the Seneca Foundation, Murcia Regional Government Agency for Science and Technology (Research Grant 19463/PI/14). Our gratitude goes to Javier Mar´ ın for his invaluable help with the statistical analyses and to the participants who generously took part in our study. Finally, we thank the three anonymous reviewers for their constructive and insightful comments and suggestions and the Language Learning editor for his expert and generous guidance during the whole process. Any problems that remain are entirely our own. Correspondence concerning this article should be addressed to Olena Vasylets, Depart- ment of Modern Languages, 585 Gran V´ ıa de les Corts Catalanes, Barcelona, Spain 08007. E-mail:vasylets@ub.edu Language Learning 00:0, xxxx 2017, pp. 1–37 1 C  2017 Language Learning Research Club, University of Michigan DOI: 10.1111/lang.12228