Purpose: To effectively implement a response to intervention approach, there is a need for timely and specific information about student learning in response to treatment to ensure that treatment decisions are appropriate. This exploratory study examined responsivity to a supplemental, Tier 2 vocabulary intervention delivered to preschool children with limited language abilities. Method: A secondary analysis of a cluster-randomized trial of a supplemental vocabulary intervention was conducted. Responsivity (e.g., adequate learning) to the intervention was examined, and learning in the 1st few weeks of intervention was evaluated as a possible predictor of response to intervention. Results: Using a criterion of learning of 20% of target vocabulary, nearly one third of participants were identified as poor responders. A 1st unit benchmark was identified that maximized the sensitivity to identification of children who were likely to respond to the intervention. Conclusions: Even for generally effective interventions, there is likely to be a substantial proportion of children who are not responsive. Learning in the 1st few weeks of intervention may be a useful indicator of appropriate response to treatment and could inform instructional decisions. [This article was published in "Journal of Speech, Language, and Hearing Research" (EJ1168764).]