This study is about the acquisition of accusative pronominal complements of third person in Brazilian Portuguese (BP) and Rio-Platense Spanish (RS). The dissertation is based in a theoretical proposal that aims for articulation between linguistic theory and processing theory in the study of Language Acquisition. The working hypothesis in this research is that the information coming from the phonic interface between language and performance systems is processed differently in each language, depending on their respective pronominal systems. We also assume that, once a children represents pronominal complements in his lexic as a set of phifeatures, there is no distinction between the two languages regarding semantinc interpretation and identification of the referent. We relate two experiments.The first one uses the headturn preference procedure, aiming to verify the sensibility of 12-22 months old children acquiring RS to the phonic form of accusative clitics. The second is a contrastive study of BP and RS, using the technique of object identification from stimulus sentences, aiming to measure the ability of 2-4 years old children to distinguish the pronominal system of their language and iterpret information relating gender and number in the identification of the referent of pronominal complements.The results suggest that children around 18 months are sensitive to the form of clitics in RS, and 2 years old children are able to distinguish the complements of their own language, although processing the pronominal reference presents difficulties even at age 4. The results are consistent with the hipotesis of this dissertation.