The interaction between double-stranded (ds) T2 DNA and the fluorescent dyes YO (oxazole yellow) and YOYO (dimer of oxazole yellow) has been studied with optical spectroscopic methods. Flow linear dichroism (LD) spectra of YO-DNA and YOYO-DNA complexes show that, at mixing ratios dye:DNA base below 0.20 and 0.125 for YO-DNA and YOYO-DNA, respectively, the long axis in the YO chromophore is approximately perpendicular to the DNA helix axis, an orientation consistent with intercalation. This conclusion is supported by the induced negative circular dichroism (CD), the transfer of energy from the DNA bases to the bound YOYO, and the unwinding of supercoiled DNA by YOYO. At higher mixing ratios, a CD exciton appears and the steady increase in the reduced linear dichroism amplitude during the intercalation phase is changed to a decrease, suggesting that a second binding mode starts to contribute. For YO, the exciton has the expected pattern for dimeric interaction between chromophores bound to the surface of the DNA. For YOYO, the new binding mode appears at a mixing ratio where, if the dye is assumed to follow the nearest neighbor exclusion principle, all intercalation sites are filled up. Thus, this second binding mode is proposed to be an external binding mode. Fluorescence anisotropy measurements show that the limiting anisotropy value (mixing ratio dye:base --> 0) for YO-DNA is about twice that of YOYO-DNA. In addition, the decrease in fluorescence anisotropy with the mixing ratio is much stronger for YO-DNA compared to YOYO-DNA. These observations have been explained in terms of depolarization of the emission due to Forster energy transfer between the intercalated chromophores.