Water boils at 100 C so how can you have a pH at 800 C? Or maybe you have a typo or not used the upper case in RG,  Do you mean 800C?  Sulfur is spelled thus.  When you do not frame your question adequately, then you are less likely to get a reasonable response especially from those with timeshare on a neuron, such as I.  Look at phase diagrams such as http://www.minsocam.org/msa/OpenAccess_Publications/MSA_Special_Papers/MSA_SP_3/MSA_SP3_199-210.pdf

Research: Pourbaix diagrams e.g. https://en.wikipedia.org/wiki/Pourbaix_diagram

There's an excellent teaching tool from the University of Cambridge here:

http://www.doitpoms.ac.uk/tlplib/pourbaix/prereqs.php

These will indicate stabilities of various phases.  So the overall answer to your question is 'it depends' - it depends on the element of interest, the oxidation states it can form, the temperature, pH as well an a number of other things.  Take a look too at the excellent textbooks  

• Gilchrist, J. D. 'Extraction Metallurgy' Pergamon Press (1967)

B.A. Wills 'Mineral Processing Technology: An Introduction to the Practical Aspects of Ore Treatment and Mineral Recovery' Pergamon Press (1988)  [Now I believe in 7th Edition]

Hi ,

your question it is not clear.In general you can get the reduction product for your oxide and one of sulfur oxides, as the temperature raised up.

 I am sorry for unclear question. Here I wanted to convert metal (molybdenum) oxides to sulfides. Do I have any possibilities of getting molybdenum sulfides when I heat Molybdenum oxide with sulfur source at higher temp.

OK, now you need to find out about Ellingham diagrams.... Also research ResearchGate and similar questions.  I can find very quickly this pertinent paper: https://www.researchgate.net/publication/269392736_Thermodynamic_aspects_during_the_processing_of_sintered_materials

See: http://chemiday.com/en/reaction/3-1-0-7618 where it states: Molybdenum(IV) oxide reacts with sulfur to produce molybdenum disulfide and sulfur dioxide. This reaction takes place at a temperature near 600°C

MoO2 + 3S = MoS2 + SO2

You still have not answered my 2 points:

• "temp varying from room temp to 800C"
• 'sulfer'

Thus I end my involvement in this question.

Article Thermodynamic aspects during the processing of sintered materials

I am sorry sir, please forgive my blunder. Yes I was referring to 800 0C and I spelled sulfur wrongly as sulfer. Thank you so much for helping me out.