Co-combustion of methane (CH

4

) and acid gas (AG) is required to sustain the temperature in Claus reaction furnace. In this study

oxy-fuel combustion of methane and acid gas has been experimentally studied in a diffusion flame. Three equivalence ratios (ER=1.0

1.5

2.0) and CH

4

-addition ratios (CH

4

/AG=0.3

0.5

0.7) were examined and the flame was interpreted by analyzing the distributions of the temperature and species concentration along central axial. CH

4

-AG diffusion flame could be classified into three sections namely initial reaction

oxidation and complex reaction sections. Competitive oxidation of CH

4

and H

2

S was noted in the first section wherein H

2

S was preferred and both were mainly proceeding decomposition and partial oxidation. SO

2

was formed at oxidation section together with obvious presence of H

2

and CO. However

H

2

and CO were inclined to be sustained under fuel rich condition in the complex reaction section. Reducing ER and increasing CH

4

/AG contributed to higher temperature

H

2

S and CH

4

oxidation and CO

2

reactivity. Hence a growing trend for CH

4

and AG t

o convert into H

2

CO and SO

2

could be witnessed. And this factor enhanced the generation of CS

2

and COS in the flame inner core by interactions of CH

4

and CO

2

with sulfur species. COS was formed through the interactions of CO and CO

2

with sulfur species. The CS

2

production directly relied on reaction of CH

4

with sulfur species. The concentration of COS was greater than CS

2

since CS

2

was probably inhibited due to the presence of H

2

. COS and CS

2

could be consumed by further oxidation or other complex reactions.