The present study addresses L2 variability/optionality as an interface problem (e.g., Sorace 2005). The authors carried out an experiment which focused on the different interpretative aspects of the Spanish personal preposition a with a group of late Spanish-English bilinguals (N=16) and late Spanish-Catalan bilinguals (N=18), in addition to a Spanish monolingual group (N=16). Besides a sociolinguistic background questionnaire, all participants took part in a completion task which tested several factors that determine the distribution of the personal preposition a (Torrego 1998, Zagona 2002). Results show omission and substitution of a and group and individual variation depending on the bilingualism scenario of the participants, supporting the difficulty in acquiring structures involving the interfaces.