We report the observation of thed 3Πg→a 3ΠuSwan bands of theC2molecule both in absorption and emission in aH2/Ar/CH4microwave discharge plasma. The input mole fraction of methane is varied from 1% to 33%. From the observed absorptions, we calculate the column density of gas phaseC2.The calculated concentration ofC2is higher in discharges containing large fractions of argon than in discharges containing large fractions of hydrogen. These observations are useful in understanding the contribution of theC2molecule to the gas phase chemistry of the microwave plasma-assisted chemical vapor deposition of diamond.