The relative transition dipole moment function M(R) for the 4 3Σ+ → a 3Σ+ electronic transition in the sodium–potassium molecule (NaK) has been determined by fitting experimental bound–free and bound–bound emission spectra. The fit is performed using a modified version of the BCONT computer program, which was originally developed by Le Roy. Spectra previously measured in this laboratory from low-lying ro vibrational levels of the 4 3Σ+ state and new spectra from high-lying levels are included in the fit. A slight adjustment to the inner, repulsive wall of the 4 3Σ+ potential leads to an improved form for that curve. The fitted M(R), when appropriately scaled, agrees very well with recent ab initio calculations of Magnier et al.