CD and uv‐visible absorption studies with several tetracationic water‐soluble porphyrin derivatives show that some of these species can serve as probes to discriminate between A‐ and B‐conformational forms of single‐stranded polynucleotides. It is also observed that these porphyrins can participate in the formation of double helices by forming transient intermediate complexes enroute to duplex formation. © 1994 John Wiley & Sons, Inc. Copyright © 1994 John Wiley & Sons, Inc.