Markit said the flash estimate of the composite index that incorporates indexes for manufacturing and service, came in at 51.5, down from 52.2 in September, which was a 27-month high.
The flash index could be revised later. It incorporates 85 percent of the data that goes in the final Purchasing Managers Index estimate.
The estimate for September came below expectations. Economists had forecast the composite PMI to come in at 52.5.
Markit said the flash estimate of the manufacturing PMI for the eurozone came to 51.3, ahead of the 51.1 mark from September, but also shy of expectations. Economists had forecast 51.4 for the manufacturing index.
The flash estimate PMI for the service sector in the eurozone came in at 50.9, down from 52.2 in September.
"The dip in the PMI in October is clearly disappointing, but it would be unwise to read too much into one month's data," said Markit Chief Economist Chris Williamson.
"It's too early to say that the recovery is losing momentum. More important is that the survey data have been running in positive territory for four consecutive months now and indicate that the eurozone economy expanded at a quarterly rate of 0.2 percent at the start of the fourth quarter, suggesting an ongoing, albeit sluggish, recovery," he said.