The J‘ dependencies of energy and line width of the 23Na39K B1Π(v‘ = 30−43, J‘) ← X1Σ+(v‘ ‘ = 2, J‘ ‘) transitions are measured up to the breaking-off points, where NaK dissociates to the Na(3s2S1/2) + K(4p2P3/2) atoms. Line broadenings are observed for transitions to the B1Π(v‘ = 30, J‘ ≥ 42), (v‘ = 31, J‘ ≥ 35), (v‘ = 32, J‘ ≥ 27), (v‘ = 33, J‘ ≥ 14), and (v‘ ≥ 34, all J‘) levels, and are attributed to the predissociation via the c3Σ+ state to the Na(3s2S1/2) + K(4p2P1/2) atoms. The (v‘, J‘) dependence of the predissociation threshold is attributed to the potential barrier due to rotation. Below and near the threshold, a series of the perturbation centers which converge to the predissociation threshold is observed for each v‘, and the perturbing state is identified as the c3Σ+ state. Rotational perturbations are observed also above the predissociation threshold, and the perturbing state is identified as the b3Π1 state. The line widths are observed to change drastically around the maximum perturbation, and this is identified as originating from the interference effect which arises because both the B1Π and b3Π1 states interact with the dissociative continuum of the c3Σ+ state. In the transitions to levels near the breaking-off points of the B1Π(v‘ ≥ 37), the line splittings into two lines are observed for each J‘. This splitting is identified as originating from the S-uncoupling interaction between the B1Π and b3Π states at a long internuclear distance. Similar line splittings are observed for the B1Π(v‘ = 30, all J‘) levels, but are not observed for v‘ = 31−36. An accidental coincidence of the level energies of the B1Π(v‘ = 30) and b3Π(v) levels is presumed, and the origin of the line splitting is identified as the S-uncoupling interaction. This is confirmed by the analysis of the hyperfine structures observed for the split lines.