The photoinduced reversible process of exciplex formation and decay between the water-soluble cationic metalloporphyrin 5,10,15,20-tetrakis[4-(N-methylpyridyl)] (Cu(T4MPyP)) and calf-thymus DNA has been studied by a picosecond time-resolved resonance Raman (ps-TR3) technique. For a detailed analysis of the exciplex properties, the following model compounds have also been investigated: double-stranded polynucleotides poly(dA-dT)(2), poly(dG-dC)(2), and poly(dA-dC). poly(dG-dT), single-stranded poly(dT), and the 32-mer d[(Gr)(7)ATAT(GC)(7)](2). Additional Raman measurements have also been done in using cw and 20-ns laser sources. It is shown that this reversible exciplex is formed, with a yield depending on the nucleic base sequence, in less than 2 ps after photoexcitation, between photoexcited ru(T4MPyP) and C=O groups of thymine residues in all thymine-containing sequences of nucleic acids. Such a rapid exciplex building process implies that it involves porphyrin molecules initially located, in the steady state of this interaction, at AT sites of the nucleic acids. This has two main consequences, which contradict previously reported assumptions (Strahan et al,, J. Phys. Chem. 1992, 96, 6450): (i) although the binding mode of the porphyrin actually depends on the base sequence, there is no preferential binding of ru(T4MPyP) to the various sites of DNA, and (ii) there is no photoinduced ultrafast porphyrin translocation from GC to AT sites of DNA. In addition, it is shown that with surrounding water molecules an exciplex can also be formed in similar to1 ps, whose spectral characteristics are not distinguishable from those formed with thymine residues. However, these two exciplex species can be distinguished from each other by their relaxation kinetics: the lifetime of the exciplex formed with water lies in the 3-12 ps range, while that of the exciplex formed with nucleic acids lies in the nanosecond time domain (1-3 ns). A set of possible routes is discussed for each of the exciplex building/decay processes.