Gaps in language skills by socio-economic status (SES) are already evident before school entry, and these gaps may change over time. After discussing mechanisms of cumulative advantages (‘Matthew effects’) and compensatory effects as well as the relevance of cultural capital and child-related activities in families, this paper tests mechanisms behind changing SES gaps in language skills from age five to nine in Germany. Analysing data from the German National Educational Panel Study with growth curve models, we find widening SES gaps in children’s vocabulary. Children of mothers with low educational attainment show a far below-average increase in skills. The findings are in line with cumulative advantage by status, although initial skills predict their growth over time as well. There are no signs of any type of compensatory effects. Reading aloud to children appears to substantially impact and mediate SES differences in vocabulary progress.