His main focus was an investigation of plant remains in coals and oil shales.

[1] In 1911, Zalessky described a new type of petrified wood from the Donets Basin in Ukraine.

In the 1960s, it was demonstrated that the fossil wood known as Callixylon and the leaves known as Archaeopteris were actually part of the same plant.

Zalessky described oval bodies of kerogen in kukersite which by his conclusion were the remains of an extinct microorganism, which he called Gloeocapsamorpha prisca.

This conclusion was criticized in the 1950s but later studies by using electron microscope confirmed Zalessky's observations.