Sequence-selective recognition of double-stranded (ds) DNA by homopyrimidine peptide nucleic acid (PNA) oligomers can occur by major groove triplex binding or by helix invasion via triplex P-loop formation. We have compared the binding of a decamer, a dodecamer and a pentadecamer thymine-cytosine homopyrimidine PNA oligomer to a sequence complementary homopurine target in duplex DNA using gel-shift and chemical probing analyses. We find that all three PNAs form stable triplex invasion complexes, and also conventional triplexes with the dsDNA target. Triplexes form with much faster kinetics than invasion complexes and prevail at lower PNA concentrations and at shorter incubation times. Furthermore, increasing the ionic strength strongly favour triplex formation over invasion as the latter is severely inhibited by cations. Whereas a single triplex invasion complex is formed with the decameric PNA, two structurally different target-specific invasion complexes were characterized for the dodecameric PNA and more than five for the pentadecameric PNA. Finally, it is shown that isolated triplex complexes can be converted to specific invasion complexes without dissociation of the Hoogsteen base-paired triplex PNA. These result demonstrate a clear example of a 'triplex first' mechanism for PNA helix invasion.