Individual differences (IDs) largely contribute to success in adult second language attainment (e.g., Dörnyei, 2006). Heritage language (HL) studies have also explored the role of IDs, namely psychosocial variables, and biographical factors with an adult HL learner population. However, the specific contribution of these variables to HL learners' performance on linguistic tests that differ in degree of explicitness and modality remains unknown. Therefore, the current study tested 103 adult HL learners of Spanish who completed a spoken elicited imitation task (EIT) and a written untimed grammaticality judgment task (UGJT) that elicited their knowledge of vulnerable morphosyntactic structures in HL bilingual acquisition. To investigate the contribution of individual learner factors on their performance, participants completed a few questionnaires. Mixed-effects regression models revealed that sequential bilingual status, willingness to communicate, generation and motivation contributed significantly, but yet differentially to participants' performance on grammatical and ungrammatical items of the EIT and UGJT.