Writing in a language different from one's mother tongue is a daunting task. The same challenge may apply to languages that have diglossic features whose spoken form differs from the written form. This article investigates Arab students' writing behaviors (fluency, pauses, and revision) in response to an argumentative composition in their L1 (first language: Arabic) and L2 (second language: English), given that understanding their complexities in writing processes would help instructors to rectify language writing-related problems. Guided by Kellogg's model, this article attempts to investigate the cognitive processes underlying these writing behaviors as aided by a keystroke logging program (Inputlog 7.0). It also examined if writing behaviors would be correlated to the writing quality of their produced final texts. Data were collected from log files of the Inputlog generated upon the students' writing processes, screened video recordings, and a stimulated recall interview. Results indicated that compared with L2 writing in character production, L1 writing processes were highly significant, less significant in pauses over word boundaries, and highly significant in time on the task factor. Concerning revision behavior, no significant differences were found in time spent on deletion and insertion, whereas a significant difference was found in R-burst for L2 writing. As regards quality of text, many pauses and minimal production of words tend to be negative indicators for writing quality output. Pedagogical implications and suggestions for future research are highlighted.