Friday's court decision opens the way for a civil case which could lead to payment of some 2.7 million euros ($2.9 million) in damages and interest plus a further million in court costs.
Montedison, now known as Edison, stood accused of burying as much as 250,000 tons of toxic and industrial waste at a facility which opened in the 1960s. Tests showed the waste had leaked into the water supply between 2004 and 2007, when forest rangers and ecologists reported finding traces of contamination.
An initial case saw the executives acquitted in December 2014.
The case harked back, albeit on a smaller scale, to an earlier scandal dubbed "Terra dei Fuochi" (Land of Fires), which saw a Mafia clan illegally and profitably dump millions of tons of toxic waste near the southern city of Naples over almost 20 years.