The present dissertation intends to investigate the syntactic nature and L2-acquisition of passive sentence structure in two Romance varieties, Italian and Spanish. First, I will present an analytic proposal couched within the Generative Approach (Chomsky, 1957, and subsequent work); from there, and following Perlmutter and Postal's (1977) consideration on the universality of the main features of passivization, the analysis will be extended to English, in an attempt to test its cross-linguistic validity. To complete the theoretical framework that lays down the groundwork for this research, the acquisition of non-linear structures will be presented throughout Chapters Two and Four, in an effort to gather a well-rounded picture of processing difficulties that are typically encountered by intermediate L2-learners (L1-English) of Romance languages when they first approach either word-order or thematic inversion. The experimental portion of the present work is presented in Chapters Five, Six, and Seven: 58 intermediate-level learners of Italian and Spanish have completed two tasks in their respective L2, with the goal of gathering information on non-linear sentence comprehension. In the first task, students were presented with aural stimuli containing pre-verbal accusative (non-subject) clitics, and passive sentences, and they had to select the picture that best represented what they heard. For the second task, students read 100 sentences divided into segments that would appear in window-format and non-cumulatively. Results indicate that non-linear structures are a challenge for intermediate L2-learners, especially when syntactic-semantic manipulations on the input yield ambiguity; nonetheless, the presentation of written stimuli fosters better performance, highlighting the importance of multi-modal approaches to language testing (Torres, 2022). [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]