This study provides an Optimality-Theoretic analysis of the syllabification of word-initial biconsonantal clusters in the productions of Faifi Arabic and Asiri Arabic speakers. This study aimed at investigating the role of sonority in the syllabification of onset clusters. Two groups, each made up of 15 participants, were employed in this study to produce English nonwords, which had onsets composed of biconsonantal clusters with different sonority levels. The results of the study showed that the two groups had two different ways of treating the clusters. The Faifi group epenthesized a vowel before the onset clusters, forcing the second consonant in the cluster to become the onset of the following syllable. The Asiri group epenthesized a vowel between the consonants of onset clusters when the sonority slope equaled 2. When the sonority slope equaled 3, the cluster was produced intact. All of these differences were shown to be the result of different rankings of several markedness and faithfulness constraints.