Shortly after ECB President Mario Draghi announced on Thursday that "the council of the ECB is ready and willing to take action, and will use all its tools," the Dax index of Germany's biggest companies jumped 1.55 percent to 10,396.66 points.
Investors had been keenly watching the ECB decision, with some worrying that the bank wouldn't continue policies aimed at keeping the European economy more liquid.
But some German economic observers warned that the ECB could never by itself ensure economic growth.
"It will never be enough for the markets. For the real economy, continuing the present bond-buying programme will have little effect," said Carsten Brezski, chief economist at ING Diba.
"This looks much more like a desperate measure."
And Professor Stefan Kooths of the Kiel Institute for the Global Economy (IW) warned in a statement that the ECB keeping interest rates low presented its own dangers.
"The risks of zero-interest policies are becoming greater with every month. Low returns are driving investors to more and more risky bets, there is a threat of systematic bad investments," Kooths said.
"The policy of ultra-cheap money is contributing little to overcoming the Euro crisis, which continues to smoulder – the problem is being put off, not resolved."