The results of a low temperature Stark-effect study on the visible short axis polarized singlet transitions of tetracene and pentacene are presented and discussed. The full anisotropy of the Stark effect could be obtained reliably only by using bioolar electric field modulation together with an optical multichannel detection system. This system is also briefly described in this paper. Using the anisotropic Lorentz approximation the difference polarizability tensor components are found to be: for tetracene ΔαLL= 29 ± 3 Å3, ΔαMM= 25 ± 4 Å3, ΔαNN= 5 ± 5 Å3; and for pentacene ΔαLL= 53 ± 4 Å3, ΔαMM= 90 ± 8 Å3, and ΔαNN= -11 ± 9 Å3. The paper emphasizes that these numbers, due to the inherent approximations of the Lorentz approximation, should be handled with great caution.