Résumé : A spin-parity analysis of the K+π- system produced by charge exchange in the reaction K+n → K+π-p is performed with data at 4.6, 8.25 and 12 GeV/c for M(K+π-) < 1.6 GeV. The unnaturalparity exchange contribution dominates for t′ < 0.3 (GeV)2. Outside the K*(892) and K*(1420) regions there is little contribution from the J p = 1- and J p = 2+ states, respectively. The S-wave J p= 0+ represents the major contribution for M(Kπ) < 1.6 GeV. It appears as a broad and smooth bump centred around 1250 MeV, but some structure may be present around 1350 MeV at 8.25 and 12 GeV/c. There is no evidence for a structure in this S-wave under the K*(892). © 1977 Società Italiana di Fisica.