Interactive book reading (IBR) as a context for vocabulary learning is studied widely in preschool children and kindergarteners. In the present study, we elaborate on this prior research by investigating the effects of a five-week IBR intervention on first graders' expressive target vocabulary growth. A repeated measures design with two experimental conditions (researcher-imposed target words vs. teacher-selected target words) was set up. Participants are 240 children (age M = 5.8, SD = 0.55) and their 18 first-grade teachers from seven different schools in Flanders (Belgium). Repeated measures ANOVA revealed significant positive effects of IBR on first graders' expressive target vocabulary for both reading conditions, measured via the TEVOC test. No differences between both conditions were found. In follow-up analyses, the characteristics of the students and the targeted words were taken into account to study potentially differential intervention effects. The age and gender of the students were not of any influence. Non-native speakers, however, benefitted significantly less from the picturebook reading activity. As to the characteristics of the target words, the age of acquisition (AoA) and tier of vocabulary were taken into account. No differences were found regarding the impact of IBR on learning low, medium, and high AoA words and the effects of the intervention were comparable for the included tier 2 and tier 3 words. These findings indicate that IBR is an excellent context to teach challenging and rare or domain-specific vocabulary and point to the importance of further investigating how to adapt IBR to the needs of non-native speakers.