The work function of tetrahedral amorphous carbon (ta-C) has been measured by Kelvin probe to lie in the range 4–5 eV, irrespective of its sp3 content or nitrogen addition. This implies that the surface barrier to emission is dominant and that emission changes caused by sp3 bonding or nitrogen addition are not directly due to changes in work function. Hydrogen, oxygen, and argon plasma treatments are all found to increase the emission of a-C, but hydrogen and argon treatments are found to reduce the work function while oxygen treatment increases it. Detailed studies of the surface with varying plasma treatment conditions suggest that the changes in emission arise mainly from changes in the surface microstructure, such as the formation of sp2 regions within the sp3 bulk. The need for local field enhancement mechanisms to account for emission over the sizeable barrier is emphasized, which may arise from local chemical nonhomogeneity, or formation of nanometer-size sp2 clusters embedded in an sp3 matrix.