The interaction of counterions with a suitably long, charged oligomer appears susceptible to treatment in the context of polyelectrolyte theory by the introduction of an end‐effect parameter that reflects the reduced association of counterions with the terminal regions of the oligo‐ion. Use of a physically reasonable value for the end‐effect parameter provides excellent agreement between theory and the experimental data of Elson, Scheffler, and Baldwin [J. Mol. Biol. 54, 401–415 (1970)] on the dependences of melting temperature on salt concentration and chain length for a series of hairpin helices formed by d(TA) oligomers. The differences in behavior expected for hairpin, dimer, and oligomer‐polymer helices are discussed. The salt dependence of the end‐joining equilibrium investigated for λ DNA by Wang and Davidson [Cold Spring Harbor Symp. Quant. Biol. 33, 409–415 (1968)] is treated as an oligomer–polymer interconversion. The dependence of equilibrium constant for this reaction on counterion concentration is in good agreement with that predicted by theory for an end‐region totalling 24 nucleotides, the known length of the λ ends.