The bank purchased €2.67bn (£2.26bn) worth of bonds, up from €1.97bn the week before, but far short of the €5bn analysts had been expecting.
However, the ECB warned the true figure could be higher as transactions take two to three days to close.
The ECB does not break down its purchases by region but it is understood to be concentrating exclusively on embattled Eurozone countries Ireland, Greece and Portugal.
While the purchases have helped to bring down bond yields in these countries, the ECB is understood to be worried about the amount it is investing.
The level of investment remains far below the €16.5bn spent in the first week of the bond-buying programme in the spring, although it is the biggest weekly amount since late June, when the ECB bought more than €7bn a week on average.
ING analyst Martin van Vliet said: “We now have a more complete picture of the scale of the ECB’s intervention in peripheral bond markets in recent weeks and, frankly, it is less impressive than we had expected.
He added: “Looking ahead, we suspect that the amount of bond buying will be scaled back substantially in the remainder of this year, amid thinning trading volumes. But early next year, when the market returns to normal trading conditions, the ECB will likely again have to step up its role as ‘peripheral bond investor of the last resort’.”
Roger Bootle at Capital Economics raised concerns the ECB might withdraw its unconventional policy measures too quickly. He said it “should try to reassure markets by providing a clear exit strategy from the existing emergency measures that are currently in place”.
Meanwhile, Angela Merkel appears to have forced the issue of eurobonds off the agenda at a crunch EU summit to discuss the financial crisis on Thursday.
The German chancellor has come under intense pressure from businesses in her home country to resist the possibility of joint Eurozone government bonds, with many fearing Germany will be left to shoulder the cost of its weaker rivals.
EU leaders will instead use the meeting in Brussels to try to set out changes to the bloc’s treaty and agree on a permanent mechanism for defending the euro against financial upheavals.
There is pressure for the European Financial Stability Facility, used to bailout troubled nations, to be extended from its current €750m but this has also been resisted by Germany and allies including Holland, who say it is sufficient.