Production errors made by second language (L2) learners of English have been attributed to markedness, L1 transfer or input frequency (cf. Major, 2001; Edwards & Zampini, 2008; Baptista, Rauber, & Watkins, 2009). This thesis examines the production of 17 English initial consonant clusters (e.g., /pr/ in “pray”) in a markedness relationship, whereby clusters with greater sonority distance between the first and second consonants are unmarked and clusters with smaller sonority distance between the first and second consonants are marked, by two groups of Saudi Arabian L2 English learners. It also explores the effect of input frequency and L1 transfer. Participants were asked to read 60 sentences and their reading was recorded for acoustic analysis. Analysis showed that “prothesis” was always used to simplify the clusters, and that, the duration of the prothetic vowel tended to get longer when clusters become more marked. Intermediate participants had greater degree of difficulty in producing the clusters and tended to insert a longer prothetic vowel in general. Markedness explained the performance on #sC clusters; however, performance on non #sC clusters was best explained by L1 transfer. Results further indicated that input frequency was irrelevant to this study.