The absorption and fluorescence properties of five polyacenes, including hexacene, are studied both theoretically and experimentally (293°C). Because of photochemical instability, hexacene was not studied before. The quantum yields of fluorescence, γ, and the decay times, τ f, of non-deaerated and deaerated cyclohexane solutions are measured. The oscillator strength, f e, of 1A→ 1L b and of 1A→ 1L a transitions, the fluorescence rate constant, K f, natural lifetime, τ 0 T, intersystem crossing, K ST, and internal conversion, K S, rate constants are calculated. The singlet 1B b, 1L b, 1L a and triplet 3L a (77°K) levels, Stokes shift, Δ ν St, and full width at reciprocal ‘e’, FWRE, of the lowest absorption bands and fluorescence spectra are experimentally determined. Systems of singlet and triplet levels are simulated (PPP-CI) and analyzed. It is found that f e of S 0→S p transition in the row of benzene-hexacene is steadily decreasing while f e of S 0→S α transition and K S values are steadily increasing. FWRE of the p-band and of fluorescence spectrum beginning from anthracene are decreasing steadily to hexacene. The same is true for K f, γ, ν 00 and Δ ν St values. Parameters τ f and K ST values behave irregularly. From benzene → naphthalene K ST decreases, then it increases significantly, by a factor of 45 (naphthalene → anthracene). In the row anthracene → tetracene → pentacene it decreases and then again increases (pentacene → hexacene). In the row of compounds benzene-hexacene τ f changes as 31.0 → 96.4 → 4.4 → 6.0 → 7.0 → 1.5 ns. The irregular behaviour of K ST value is explained by considering the change in the nature of S 1 state and in the position of 3B b( 3B 2u +) level. The quantum yields of fluorescence, intersystem crossing and internal conversion of pentacene and hexacene are found to be 8, 76, 16, and 1, 94.5, 4.5% respectively.