We have studied the mutually perturbing 33ΠΩ=0(v = 32, J = 19) ∼ 31ΠΩ=1(v = 6, J = 19) levels of NaK that are coupled together by the spin–orbit interaction. We note that this coupling is nominally forbidden by the ΔΩ = 0 selection rule for spin–orbit perturbations. However 33Π levels labeled by different values of Ω are mixed by rotational coupling; i.e. the 33ΠΩ levels are best described by a coupling scheme intermediate between Hund’s cases (a) and (b). Thus the 31ΠΩ=1 level couples to the 33ΠΩ=0 level via the small admixture of 33ΠΩ=1 character in the latter. The 33ΠΩ=0(v = 32, J = 19) ∼ 31ΠΩ=1(v = 6, J = 19) f symmetry pair is of particular interest since it appears to be very close to a 50–50 mixture of triplet and singlet character, and the splitting between these levels provides a direct measure of the 33Π ∼ 31Π spin–orbit coupling constant. On the other hand, excitation spectra of the 33ΠΩ=0(v = 32, J = 19) ∼ 31ΠΩ=1(v = 6, J = 19) e symmetry pair through the mixed “window” levels 1(b)3ΠΩ=0(v = 17, J = 18, 20) ∼ 2(A)1Σ+(v = 18, J = 18, 20) display dramatic quantum interference effects associated with “singlet” and “triplet” excitation channels. Almost complete cancellation for populating one or the other of the two upper states is observed for excitation from the predominantly triplet members of the window level pairs.