This study analyzed the evolution of Interlanguage in the writings of intermediate and advanced level learners of Arabic. The analysis was conducted by using Larry Selinker’s (1972 and 1992) definition of Interlanguage, a new language system that is related to but also distinct from both the learner’s L1 and L2. This exploratory, descriptive study comprised of an analysis of 100 expository texts from university students enrolled in second-, third-, and post-third year Arabic courses, ranging from Intermediate Low to Advanced High language proficiency levels. The data were analyzed both qualitatively and quantitatively and looked at the following cohesive features: the use of overt and zero connectives, instances of repetition, lexical cohesion, and language transfer from English as a way of measuring the development of Interlanguage in the participants’ Arabic writings. Each cohesive feature in the writing samples was measured by the number of occurrences, variety, and quality at the clause, sentence, paragraph, and discourse levels. The goal was to investigate the features and measure the development of the written Interlanguage of Arabic language learners as they progress in their written proficiency. The results of this study indicate two important aspects of Arabic Interlanguage development in writing among Arabic language learners: 1) Arabic students go through phases in the development of their Arabic writing skill and, 2) the transition from phase to phase indicates that Interlanguage development in writing is not a linear process. In this study, second- and third year participants varied in their variety and frequency of overt and zero connective. Post-third year participants demonstrate an awareness of the importance of connectives as cohesive markers. Overall, participants in this study are developing their application of connectives, yet their writings, at times, still contain drops and English-style sentences. Regarding lexical cohesion, second- and post-third year participants used lexical couplets, yet third year participants did not. Additionally, each phase featured an increase in the frequency of reiteration and reference, specifically anaphora, among participants as they move from second- to third- to post-third year. Overall, the findings from this study offer insight into the features of Interlanguage development in writing among Arabic language learners.