Borax has been identified as a possible crystallization modifier for sodium sulfate. Understanding the effect of borax on factors influencing transport and crystallization kinetics of sodium sulfate helps to clarify how this modifier might limit crystallization damage. It has been observed that the addition of borax to sodium sulfate solutions has no influence on the wetting properties (contact angle on glass, surface tension, or evaporation rate) and therefore will not influence solution transport. Additionally, the influence of borax on the crystallization kinetics of sodium sulfate was studied under controlled environmental conditions. This was carried out in mixtures in glass microcapillaries, and sequentially in droplets on glass plates. Under the here studied precipitation conditions, the addition of borax has no influence on the supersaturation ratio at the onset of crystallization, but it significantly affects the crystallization pattern of anhydrous sodium sulfate crystals (thenardite). Using RAMAN spectroscopy, two different hydrates of borax were identified after precipitation, depending on the initial concentration of the solution. Each hydrate has a different effect on the subsequent crystallization of sodium sulfate. The decahydrate polymorph of borax leads to the precipitation of hydrated sodium sulfate crystals (mirabilite) and the pentahydrate form favors the precipitation of the anhydrous sodium sulfate crystals (thenardite) with an altered crystal habit. Using X-ray diffraction, overdevelopment of the (111), (131), (222) and (153) faces of thenardite was identified. Additionally, the ratios between several peaks are reversed. These results confirm the deviation of the grown crystals of the equilibrium crystal shape of thenardite as observed with optical microscopy.