The excited-state dynamics of an oligomer of polydiacetylene, 2,2,17,17-tetramethyloctadeca-5,9,13-trien-3,7,11,15-tetrayne, dissolved in n-hexane have been studied by femtosecond fluorescence upconversion and polarized transient absorption experiments under one-and two-photon excitation conditions. Spectroscopically monitoring the population relaxation in the excited states in real time results in a distinct time separation of the dynamics. It has been concluded that the observed dynamics can be fully accounted for on the basis of the two lower excited states of the target molecule. The S-1 (2(1)Ag) state, which cannot be excited from the ground state with one- photon absorption, is verified to be populated via internal conversion in 200 +/- 40 fs from the strong dipole-allowed S-2 (1(1)Bu) state. The population in the "hot" S-1 state subsequently cools with a time constant of 6 +/- 1 ps and decays back to the ground state with a lifetime of 790 +/- 12 ps.