With numbers below 50 indicating a contraction, the Markit Eurozone Composite Output Index dropped from 46.5 percent in July to 46.3 percent in August, Markit said.
A previous estimate released earlier, called a flash estimate, showed business activity at 46.6 percent.
The report on the 17-member eurozone includes the August PMI for service-oriented businesses, which the survey put at 47.2 percent.
That was also lower than the July figure of 47.9 percent and the flash estimate of 47.5 percent.
The composite index measures both service and goods-producing business activity In August, "the downturn in output was again steeper in the manufacturing sector," Markit said.
Although the pace of contraction slowed to a two-month low in the manufacturing sector, the index for goods-producing firms "was still one of the steepest registered since the end of the previous recession in 2009," the report said.
Service sector activity, on the other hand, slowed for the seventh consecutive month with the contraction at a faster pace than July.
High points from specific countries include a two-month high for Ireland's composite PMI at 51.4.
In France, the composite PMI reached 48, a five-month high. In Germany, it reached 47, a 38-month low.
Italy posted a PMI of 43.5, a three-month high. Spain posted a PMI of 43.4, a five-month high.
"Sharp declines in new orders at manufacturers and service providers, plus further job losses, mean that there is little prospect of a sustained improvement in economic conditions over the near-term," Markit Senior Economist Rob Dobson said.
"Some heart can be taken from the recent recovery in Ireland, which is providing hope to others that a return to growth is possible," he said.
Also on the bright side was "further evidence that the downturns in Italy and Spain may at least be easing," he said.