Abstract An isospin analysis has been made of the production of (N π) states in the reactions π ±p → π(N π) at 16 GeV/ c. The three independent amplitudes and their three interference terms have been determined as functions of the (N π) mass. It is found that at 16 GeV/ c isospin-zero exchange is the dominant process, and isospin-one occurs much less frequently, mostly producing the I = 3 2 Δ (1236) isobar. Interference effects do not seem very important. The I = 1 2 ( Nπ) mass spectrum due to isospin-zero exchange presents a broad enhancement from 1.1 to 1.8 GeV, with a broad maximum between 1250 and 1500 MeV. The N * isobars found by phase shift analyses at 1470, 1520, 1690, etc. MeV cannot account for the whole enhancement observed and in particular for its low-mass part. These results are consistent with the interpretation that diffractive phenomena are important at low N π masses and may cause mass enahancements not revealed byphase-shift analyses.