Trioxin is a gas-liquid in the Return of the Living Dead Series that can reanimate inactive organic matter and convert living flesh into reanimated dead flesh, or zombie. There are more than one kind of Trioxin as the zombies differ in the films:
This form of Trioxin is responsible for the 2-4-5 Trioxin Zombies. In gas form, 2-4-5 Trioxin can instantly convert the living into Triombies and reanimate the dead. When in liquid form, it has a slight corrosive effect.
The Trioxin in this movie was found in the Trioxin canisters, which accidentally released 2-4-5 Trioxin into the air. Although this happening caused only two corpses to be reanimated (a cadaver and Tarman), when the cadaver was cremated the Trioxin smoke was released and was turned into liquid by the rain. This Trioxin rain fell into Resurrection Cemetery and seeped into the dead's corpses underground and reanimated them.
The Trioxin in this movie was also from a canister which was tinkered with by Billy Crowley. The Trioxin was released into the nearby cemetery and reanimated the corpses within.
This movie's Trioxin was used by the government to develop a new weapon: ever-lasting soldiers. However, the Trioxin seemed more sensitive in this movie than in the previous two because it seemed to spread by bite. Fortunately, the Trioxin did not spread throughout the town but instead just to a handful of people.
This form of Trioxin is responsible for the creation of Trioxin 5 Zombies. It is unknown if it has a corrosive effect, but it does not effect the living unless it is ingested.
The Trioxin in this movie was secretly used in an operation by Hybra Tech known as "Necropolis". Hybra Tech used this Trioxin to try and make more uber-zombies (similar to those in Return 3) but with Trioxin 5.
This Trioxin is the same in the previous movie but was manufactured as a recreational drug known as "Z".