Devolatilization characteristics of coal in oxy-steam combustion were studied.

Char characteristics during devolatilization in H₂O/CO₂ differ from those in N₂.

High concentration of H₂O greatly decreases the char yields and condenses the char.

CO₂ in H₂O/CO₂ affects char yield little but leads a less condensed char.

A relationship between the char structural features and char reactivity was set up.