The excitation temperatures of C$_2$ and CN radicals generated in a shock tube by the pyrolysis of cyanogen and acetylene have been measured in the range 3000 to 5000 $^\circ$K by a brightness and emissivity method. The experimental measurements were made with photomultipliers whose outputs were recorded on a cathode ray oscillograph, so that complete histories of the temperature could be obtained. The radicals showed a radiative relaxation zone at the front followed by a period of constant temperature whose value agreed well with the results of calculations. These allowed for the species C$_2$H$_2$, C$_2$H, C$_3$, C$_2$, C, CH, H$_2$ and H from acetylene and CN, C$_3$, C$_2$, C, N$_2$, and N from cyanogen. The reaction 2CN $\rightarrow$ C$_2$ + N$_2$ is discussed and found to have an activation energy of 96 kcal/mole. The processes leading to the formation of the electronically excited radicals are discussed with reference to the radiative relaxation at the shock front.