Attention is drawn to a class of signals that appear if the rotation angles of the radio-frequency pulses in the 2-dimensional (2D) exchange expt. are inadvertently or deliberately set to b!=p/2. The signals (zz signals) arise from longitudinal 2-spin order in the mixing period, and can be sepd. from genuine exchange signals by exploiting the characteristic response to a p pulse inserted in the mixing period. Pulse sequences are shown for (1) conventional 2D exchange spectroscopy, (2) a modified 2D exchange expt. for excitation and reconversion of longitudinal 2-spin order, and (3) suppression of zz signals in 2D exchange spectroscopy. The efficiency of the sepn. procedure is illustrated with NOESY spectra of the protein basic pancreatic trypsin inhibitor. [on SciFinder (R)]