Purpose: The purpose of this study was to determine which factors contribute to the lexical learning deficits of children with specific language impairment (SLI). Method: Participants included 40 7-8-year old participants, half of whom were diagnosed with SLI and half of whom had normal language skills. We tested hypotheses about the contributions to word learning of the initial encoding of phonological information and the link to long-term memory. Children took part in a computer-based fast-mapping task which manipulated word length and phonotactic probability to address the hypotheses. The task had a recognition and a production component. Data were analyzed using mixed ANOVAs with post-hoc testing. Results: Results indicate that the main problem for children with SLI is with initial encoding, with implications for limited capacity. There was not strong evidence for specific deficits in the link to long-term memory. Conclusions: We were able to ascertain which aspects of lexical learning are most problematic for children with SLI in terms of fast-mapping. These findings may allow clinicians to focus intervention on known areas of weakness. Future directions include extending these findings to slow mapping scenarios. Learning outcomes: The reader will understand how different components of phonological working memory contribute to the word learning problems of children with specific language impairment. (Contains 3 figures and 2 tables.)