Children who struggle with writing are a heterogeneous group and may experience difficulties in a range of domains, including spelling, reading, and oral language. These difficulties are reflected in their writing and may influence their responsiveness to writing interventions. The effectiveness of a targeted sentence-combining intervention to improve the writing skills of 71 struggling writers, aged 7 to 10 years, was compared with a spelling intervention and a business as usual (waiting list) control condition. Some struggling writers also performed poorly on measures of reading and oral language. Children's performance on a range of writing measures were assessed at baseline (t1), immediate post-test (t2) and delayed post-test (t3). Children receiving the sentence-combining intervention showed significant improvements in the sentence combining measure at t2 and t3 compared to both the spelling intervention and waiting list controls. Exploratory regression analyses found that children in the sentence-combining intervention, with a low t1 sentence combining score, low reading skills or better t1 spelling skills, were more likely to show improvements at t2. Findings indicate that when devising interventions for struggling writers, specific profiles of skills should be considered. Specifically, sentence combining may be more appropriate for SWs whose primary area of difficulty is reading, rather than poor spelling or oral language.