The photoabsorption cross section of CH3CN vapor was measured in the 106--180 nm region using synchrotron radiation as a light source. The cross section and the quantum yield for the production of CN (A,B→X) fluorescence were measured and were used to infer the photodissociation cross section of CH3Cn. The cross sections were used to calculate the photodissociation rates of CH3Cn by the solar flux and by the interstellar radiation. In both the stratosphere and the troposphere, the solar photodissociation of CH3CN is negligible in comparison with chemical degradation.