The Mexican peso erased its gains to fall about 0.4 percent after ECB president Mario Draghi said the bank plans to make outright purchases of bonds issued by troubled euro zone members, but would take a few weeks to design a mechanism for that.
The Brazilian real , which was gaining about half a percentage point before Draghi's comments, traded 0.3 percent lower after them.
Expectations that the ECB would act decisively to curb the spreading of the euro zone crisis had supported investor appetite for emerging market assets in the past few days, especially since Draghi pledged last week to do "whatever it takes" to preserve the euro.
Draghi's comments were a second major disappointment for investors this week, after the US Federal Reserve on Wednesday stopped short of offering new stimulus measures to support the economy.