Abstract: Hydrogenated amorphous carbon（a-C∶H）films are composed of SP³C,SP²C and H,the composition range of dense hydrocarbon is given in their phase diagra m. There exists the errors in IR analysis in the determination of the SP³C/SP²C ratio,leading to the deviation of the determined structures from the composition range.It concluds by analyzing the IR results that IR analysis leads to the hig her content of SP³C and the lower one of SP²C,and also that SP²C exists mainly i n the form of unprotonated carbon,whose amount can be up to 67% and the amount of protonated carbon is lower（10% of the total number of SP²C atoms）.The structure of a-C∶H films is the fully constrainted cross-linked network.