This study looks at both the knowledge and use of the Spanish pronoun se mainly with unaccusative predicates in the interlanguage of an adult English speaker learning Spanish as a Second Language (L2) in a native, non-institutional setting. Data recorded from the learner’s production along with other similar data were presented to him as a written acceptability judgment task. A comparison was made with the outcome of the task in order to have a more comprehensive view of his linguistic competence. The results seem to confirm those of previous studies in that the learner overgeneralizes the use of the clitic se. They also suggest that the development of L2 knowledge may involve a process of construction resulting in structures not present either in the native language (L1) or the L2, which show in certain fossilized errors. It is argued that these structures originate in the interaction of Universal Grammar principles and both L1 and L2 influence.