Having a clear vocabulary profile of Moroccan master students might reveal where these participants stand compared to other countries. Therefore, the study aims to investigate the effect of languages of instruction in Morocco (e.g., Arabic vs. French) on the receptive vocabulary size of EFL Moroccan master students (e.g., departments of letters, science, and law). To this end, Meara's (2010) Yes/No test was used as an instrument to measure the overall vocabulary size of these participants. A

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... al of 325 EFL master students took the aforementioned test. The main research question is: Does the medium of instruction have any effects on the receptive vocabulary size of these students? Descriptive statistics were employed to calculate the overall receptive vocabulary size of test-takers. It was found that Moroccan EFL master students have a total of (M= 2293) lemmas. An independent samples t-test was run to check for any statistical significance. The t-test statistic reveals that the significance level is less than the p-value (t=-4.068, p< .05, df= 323). Thus, it was concluded that there was a statistically significant difference between the French group and the Arabic group. The results of this study confirm that students who were instructed in French (M= 2417, sd= 903, N=185) outperformed the other students who were taught in Arabic (M= 2058, sd= 903, N= 140). In the current study, among various suggestions, it is proposed that the volume of 30 hours in the English module is not sufficient and should be complemented with vocabulary-based activities.