This data collection contains data from the first of four studies conducted on the associated ESRC grant (data from the other studies will be made available as separate datasets in ReShare). The purpose of this study was to investigate the extent to which primary memory development constrains the development of working memory in children, and whether primary memory capacity mediates the relationship between working memory and academic attainment. To that end, a sample of 101 children aged between 5 and 8 years were given three novel experimental measures of primary memory capacity that were designed to estimate the number of items in a child's immediate memory that they could spontaneously recalled in correct serial order. More traditional experimental measures of short-term and working-memory capacity were also administered, as were standardised tests of reading [Sentence Completion Forms of the NFER-Nelson (1998) Group Reading Test II Form A (6–14)] and mathematics [NFER-Nelson (1994) Mathematics 6–14]. These data underpin the following paper: Hall, D., Jarrold, C., Towse, John N., & Zarandi, A. L. (2015). The developmental influence of primary memory capacity on working memory and academic achievement. Developmental Psychology, 51, 1131-1147. doi: 10.1037/a0039464 which is available as an open access publication (see related resources section). The data are also available via the University of Bristol data repository (see related resources section).