This work studies the effects on the Bayer process of various high-molecular-weight fractions of organic matter (1200 to > 300 000 Da) derived from the high-temperature digestion of Weipa bauxite. This preliminary study shows the impacts on precipitation of aluminum hydroxide (gibbsite) and sodium oxalate under typical Bayer precipitation conditions. Different molecular weight fractions of the Bayer-degraded humic substances were found to have different detrimental effects on gibbsite precipitation yields and on crystal sizing and surface area. It is clear that certain high-molecular-weight organics, although present in low concentrations in Bayer process liquor, can have strong affects on the dynamics of gibbsite precipitation, adversely affecting yields but also lowering particle surface areas. These are significant new findings and emphasize the specific effects of Bayer organics according to molecular weight. These high-molecular-weight organics were also found to stabilize sodium oxalate in solution by significant amounts, exhibiting an increasing trend with molecular weight.