Introduction: The present study investigated English-speaking 5-9 year olds' (n = 600, normative sample) comprehension of relative, adverbial and complement clauses using the Test of Complex Syntax-Electronic (TECS-E), an online interactive assessment. with strong test-retest reliability, concurrent validity and internal consistency. Method: Using an opt out protocol three groups of children were recruited from schools--a sample for whom we could ascertain test-retest reliability (n = 122), a sample to explore concurrent validity, using the Test for the Reception of Grammar (TROG-2) (n = 131) and a main normative sample (n = 906). The main sample was reduced to 600, to ensure that the final sample accurately represented the population of the Republic of Ireland, with respect to sex, socio-economic status and locale. Children completed the TECS-E independently in groups within their pre/school classroom. The TROG-2 was administered individually. Results: Scores increased with age from 5-9 years with considerable growth in development between 5;06 and 7 years, suggesting that 7 years is an important milestone in children's understanding of complex syntax. Girls performed significantly better than boys, particularly between 6 and 7 years. Those from disadvantaged backgrounds scored significantly lower than those who were more affluent, until 7 years, and this was more marked in boys than in girls. Children performed highest on relative clauses, followed by complement clauses, and last on adverbial clauses. Conclusion: This study provides a developmental hierarchy as well as important information regarding growth in English-speaking children's understanding of complex syntactic structures in the early school years and how this interacts with sex as well as social disadvantage.