The toxic red sludge overflow from a Hungarian factory which destroyed three villages has reached the Danube, Europe's second-longest river.
As well as providing much of the country's drinking water, the river flows through Croatia, Serbia, Romania, Bulgaria, Ukraine and Moldova before emptying into the Black Sea.
The EU had feared an environmental catastrophe if the sludge, a waste product of making aluminium, contaminated the 1,775-mile long Danube.
It overflowed a reservoir on Monday and emptied into local streams that feed waterways connected to the Danube.
Those in Kolontar, 45 miles from the Danube and the closest town to the spill site, were swollen ochre red and villagers said they were empty of fish.
The sludge reached the western branch of the Danube early on Thursday. The Hungarian rescue agency did not discuss its possible toxic metal content but said its acidity had been reduced to the point where it was unlikely to cause further damage to the environment.
It said the pH content, which had hit 13, was now under 10. Normal pH levels for surface water range from 6.5 to 8.5.
At the Croatian village of Batina, the first site after the Danube leaves Hungary, experts were taking water samples on Thursday which they will repeat. In Romania, the water was reported safe, with testing being carried out every three hours.
Hungarian Prime Minister Viktor Orban visited the three villages coated by the red sludge and declared the worst-hit area a write-off, saying he saw "no sense" in rebuilding in the same location.
Residents said the disaster had destroyed the whole community of 800 by making their land valueless, and angry villagers gathered outside the mayor's office.