Background: The profiles of children with Specific Language Impairment (SLI) differ greatly according to the language they speak. The Surface Hypothesis attempts to explain these differences through the theory that children with SLI will incorrectly produce elements in their language with low phonological weights or that are produced in a non-canonical prosodic structure. Aims: Previous studies have shown that the most characteristics errors produced by Catalan and Spanish-speaking children with SLI include function word omission (morpho-syntax) and weak syllable omission (phonology). The omission of function words points to a morpho-syntactic explanation of SLI, while weak syllable omission supports a phonological explanation of SLI. Yet, function words are weak syllables; thus, it is possible that the same mechanism underlies both problems. Methods & Procedures: Data were extracted from spontaneous language produced by five children with SLI and five comparison children matched for age and MLU-w. They were assessed on two occasions: at 3;10 and 4;9 years of age. These interviews were then transcribed and the morphological and phonological errors coded. A non-parametric mean analysis and various regression analyses were conducted. Outcomes & Results: The results show that function word omission and weak syllable omission were the most characteristic errors made by Spanish and Catalan-speaking children with SLI and established that omissions increase as prosodic weight decreases. They also indicated that weak syllabic omission may explain most function word omissions. Conclusions & Implications: The data support the Surface Hypothesis and suggest that the same impaired mechanism may underlie the morphological and phonological problems SLI children display. (Contains 8 tables and 12 figures.)