In this study, a thermally-energetic Mg source with an independent, valved-flux control was used to study the behavior of Mg incorporation into GaN. To observe effects of the thermal energy of the Mg flux on Mg incorporation, two Mg flux temperatures were investigated: one (900°C) well above the melting point of Mg and one (625°C) slightly below the melting point of Mg. Alternating Mg-doped and undoped GaN layers were grown at steps of increasing Mg flux, retaining a constant thermal energy, from below the saturation limit, to above the saturation limit. Results were analyzed and compared using secondary ion mass spectroscopy (SIMS). For a constant measured Mg flux, the incorporated Mg increased by more than an order of magnitude when the Mg thermal source temperature was raised from 625°C to 900°C. During SIMS analysis, the energy spectra of sputtered Ga atoms were fairly constant for a Mg flux above the saturation limit, and shifts for a Mg flux slightly below the critical flux for saturation, indicating a conductivity change, and possibly providing a quantitative means of optimizing p-type conduction. Furthermore, Mg incorporation into GaN strongly depends on the III-V flux ratio. During this study it was also observed that Mg incorporation into GaN was enhanced on a rough growth-layer surface under N-rich conditions, while a smoother growth-layer surface resulted in lower Mg incorporation, even under N-rich conditions1.