The anode and cathode reaction processes for conventional solid oxide fuel cells (SOFC) have been widely studied. In comparison, there are many unknowns about the basic electrochemical behaviors and mechanisms for the anode and cathode reactions of proton conducting SOFC, especially at intermediate temperature (IT) range of ~400-700^oC when the cells approach ideal proton conducting behaviors. At intermediate temperature, due to the change of the dominating ionic conducting species from oxide ion to proton, the fundamental reaction pathways would change for both the anode and cathode reactions. Accompanied with such changes, many interesting electrochemical behaviors arise for proton conducting IT-SOFC. For example, it is well known that the Ni-based cermet anode for conventional oxide ion SOFC are susceptible to poisoning by sub-ppm level hydrogen sulfide (H₂S) and not very sensitive to carbon monoxide (CO). In comparison, literature seems to suggest that the Ni-proton conducting oxide cermet as the anode for proton conducting IT-SOFC shows much better resistance against low ppm-level H₂S but is poisoned by percentage level CO. This study aims to understand the fundamental reaction processes for both the anode (hydrogen electrode) and cathode (air electrode) of proton conducting IT-SOFCs. Electrochemical tests are carried out based on the concept of “controlled poisoning”, which is to selectively slow down a specific part of the electrode reaction using a specially chosen “poison” in order to help understand the fundamentals about the electrode reactions for proton-conducting IT-SOFCs. In particular, for the anode, H₂S is used as a “Ni poison” and CO₂ as a proton conducting oxide poison, while for the cathode, either H₂O or CO₂ is used. The measured electrochemical behaviors for different anode- or electrolyte-supported full cells and symmetrical cells including those with patterned (metal) electrodes are compared with those of conventional oxide ion based SOFCs. The results for those experiments aimed at understanding the similarity and differences between proton conducting SOFC and conventional oxide ion conducting SOFC will be presented, and their implications on the anode and cathode reaction mechanisms and the roles (including electrocatalytic roles) of proton conducting oxides in those electrode reactions for proton conducting IT-SOFC will be discussed.