It is well documented that children’s language skills already vary by socioeconomic status by the time they are about 2-3 years old. In addition, study results demonstrate that the frequency of language-stimulating interaction behavior – as an important aspect of the proximal familial learning environment – significantly predicts children’s later language skills. However, it is less clear how early social disparities in children’s language emerge and to what extent they are mediated by maternal language-stimulating interaction behavior. Therefore, we investigated disparities in children’s vocabulary at 26 months of age and analyzed whether and to what extent the effect of maternal education on vocabulary acquisition is mediated through language-related episodes of joint attention, as a facet of language-stimulating interaction behavior. We used data from N = 778 children of the Infant Cohort Study of the German National Educational Panel Study. The results show early disparities in children’s vocabulary development as well as in language stimulating interaction behavior in mother-child interaction according to maternal education; however, only 9 % of the eff ect of maternal education on early vocabulary development is mediated through the assessed language stimulating interaction behavior when controlling for child language skills at 14-17 months of age.