Corrosion fatigue of C-Mn steels is a hydrogen controlled phenomenon and is borne out by the significant experimental evidence which indicates an increase in FCGR with decreasing frequency. There is some amount of published data that has been generated under range of different conditions on X60/X65 and even X80. There has been significant data has been generated under differing conditions of DK Kmax R ratio’s and environmental conditions. There are several possible hypothesis for the rate controlling step in the propagation of corrosion fatigue cracks in sour environments: a) Presence of pre-soaked diffusible hydrogen in the metal b) Cracktip reactions generating hydrogen c) Combination of both a and b A set of experiments were performed to evaluate the effect of environmental variables of interest in particular pH and pH2S. The role of pre-charged hydrogen on FCGR was quantified by comparing data from tests in which samples were uncoated as opposed to coated samples. Crack tip chemistries were also measured under these conditions to determine the contribution of crack tip environments to the FCGR process. The role of internal hydrogen versus crack tip hydrogen in the FCGR process will also be discussed based on critical experiments performed.