We report the observation of the d (IIg)-I-3 --> a (IIu)-I-3 Swan bands of the C-2 molecule both in absorption and emission in a H-2/Ar/CH4 microwave 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 phase C-2. The calculated concentration of C-2 is 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 the C-2 molecule to the gas phase chemistry of the microwave plasma-assisted chemical vapor deposition of diamond.