This article examines the role of language skills in socially stratified educational attainment. Using essays written at the age of 11 in a large British cohort study, the National Child Development Study (NCDS), two measures of written language skills are derived: lexical diversity and the number of spelling and grammar errors. Results show that participants from the lower social strata misspelt more words and used a smaller variety of words in their essays than more socially privileged cohort members. Those language skills mediate part of the association between social origin and the highest level of educational attainment achieved. An even higher mediation of about half can be observed if standardised test measures for verbal and non-verbal cognitive abilities are included in the model. The models show that language skills mediate the social origin effect on educational attainment by about a quarter.