Markit's preliminary report for September released earlier showed that the headline purchasing manager's index (PMI) was 51.9. That's expected to be unchanged when the firm releases its final report at 9:45 a.m. ET.
At 10 a.m., ISM's non-manufacturing PMI is estimated at 53.
In both cases, a reading above 50 would indicate that the service sector remained in expansionary territory.
The service sector creates most of America's jobs and is responsible for most economic output, which is why it is closely watched by economists.
Wells Fargo's Sam Bullard wrote in a preview of ISM's report to clients:
In a surprising move, the ISM non-manufacturing index plunged 4.1 points in August to a six-year low of 51.4. We think the August downturn was a one-off, as the non-manufacturing sector appears to have substantial positive momentum. Indeed, the September Richmond Fed services survey showed that revenues in the sector strengthened with gains from both retailers and other services.