[en] A sample of 198 events, corresponding to a cross section of 110 +- 15 μb, has been isolated of the baryon-diffractive-dissociation process n → pπ^-π⁰ for three-body masses less than 2.5 GeV/c². These dissociations are induced by 12-GeV/c K^- mesons. To isolate this process from background, it is necessary to observe the final-state proton explicitly by its ionization in the bubble chamber. The three-body mass spectrum seems different from that of the nπ⁺π^- neutron diffractive breakup. A Δ⁰ signal is seen as well as possible N/sup asterisk/₁₄₇₀, but no clear Δ⁺ is observed. The angular distribution of the proton in the pπ⁺π^- rest frame indicates that a multiperipheral mechanism is present