Autoxidation significantly decreases the shelf life of whole milk powder due to primary and secondary oxidation products such as hydroperoxides, aldehydes, ketones, alcohols and hydrocarbons. Water soluble chitosan has been shown to reduce oxidation by chelating metals or combining with lipids resulting in a significant antioxidative effect. The objective of this study was to determine the antioxidative effects of different concentrations of low (L) and high (H) molecular weight water soluble chitosan (9 and 90 kDa, respectively) on whole milk powder (WMP). Commercially dried WMP was obtained and rehydrated 50% (wt/wt) with various aqueous chitosan solutions (2% L2, 4% L4, 2% H2, 4% H4) (L2 = Freezedried WMP with 2% low molecular weight chitosan added; L4 = Freeze-dried WMP with 4% low molecular weight chitosan added; H2 = Freeze-dried WMP with 2% high molecular weight chitosan added; H4 = Freeze-dried WMP with 4% high molecular weight chitosan added) yielding approximately 50% solids concentration. A control was prepared with no added chitosan (0%). Rehydrated WMP was freeze-dried then all samples were stored at 45°C for 48 days. Samples were evaluated for moisture content, water activity, color, thiobarbituric acid reactive substances (TBARS) and peroxide value (PV) every 8 days for 48 days. Moisture and water activity of control, L2, L4, H2, and H4 treatments did not significantly differ during storage. The addition of chitosan tended to result in lower L*, H and higher a* values. The b* values were also higher in WMP with added chitosan near the end of storage. C* values for H2 and H4 were higher than other treatments. TBARS of L2, H2 and H4 were not significantly different from control TBARS during storage except the TBARS of H2 and H4 which were higher than control TBARS on days 40 and 48 (P<0.05). PV of L2, H2, H4 was significantly lower than control PV on days 16, 24, 32 (P<0.05). TBARS and PV of L4 were significantly lower than control values on days 0, 16, 40 and 48 (P<0.05). All treatments of chitosan exhibited a significant increase in TBARS and PV (P<0.05) values during storage. However, 4% of the low molecular weight chitosan inhibited oxidation as measured by TBARS and PV when compared to all other treatments and the control.