The return of the Simon Cowell-fronted talent contest helped ITV1 maintain its share of impacts year on year among adults aged 16 to 34.
Meanwhile, it suffered only a 1% year-on-year drop in its share of all-adult impacts and a 2% drop in its share of impacts year on year among adults aged 35+.
The performance was partly boosted by the broadcaster's decision to air a second X Factor show on Sunday
evenings. The Sunday shows have helped ITV1 pull in significant new impacts - the 4 October show peaked with 13.9 million viewers, said ITV.
An ITV spokesman, however, while welcoming The X Factor boost, highlighted ITV1's performance in other parts of the schedule: "Soap and drama performed well in September. And we were helped by football. The England v Croatia World Cup qualifier on 9 September peaked on ITV1 with 13.9 million viewers."
Richard Oliver, managing partner investment, UM, said: "ITV's performance is good, although I would say it is stable rather than upward."
ITV1's performance was much better than that of Channel 4 and Five. While ITV1 broadly held its share of all-adult impacts, C4's dropped 9% and Five's slipped 10%.
Kelly Williams, director of sales, Five, said: "We had a good September last year when we aired more live UEFA Cup games than we did this September.
However, our 2009 autumn schedule is only just bedding in and we expect that to perform strongly. The 5 October episode of FlashForward picked up around seven million viewers."
Channel 4's share of adult 16-34 impacts was down 12% year on year. Mike Parker, head of strategic sales, C4, said the broadcaster remained confident of a solid performance over the rest of the year. "We're confident of the upcoming schedule," he added. "And our digital channels are increasing their share."