In this paper, vowel epenthesis in Urban Hijazi Arabic is analysed as a process of gradual structural build-up. Harmonic Serialism, a derivational framework of Optimality Theory, provides the theoretical foundation to illustrate the arguments. Rather than epenthesising an entire vowel all at once, featural structure progressively increases in successive steps. This accumulation continues until the required vowel quality is achieved. Specifically, the constraint hierarchy predicts high epenthetic vowels to occur in closed syllables and the low epenthetic vowel in open syllables. The same constraint hierarchy, however, is also expected to predict both gradual epenthesis and gradual deletion. In that regard, a seemingly paradoxical situation is created when the very same intermediate vowel quality is achieved through accumulation or attrition of featural structure. This particular vowel quality, in exactly the same environment, will have to continue gaining internal structure towards epenthesis or continue losing internal structure towards deletion. Eventually, identifying the path that the derivation takes to reach a certain vowel will help to resolve the issue.