The present invention relates to a process for preparing glucaric acid or lactone thereof, the process comprising: reacting glucose with oxygen in the presence of a heterogeneous oxidation catalyst comprising a metal selected from the group consisting of Pd, Pt, and a combination thereof to convert at least a portion of the glucose to glucaric acid or lactone thereof, wherein the pH of the reaction mixture is less than 7 and the reaction is conducted in the substantial absence of added base, and wherein at least about 50% of the glucose is converted to glucaric acid or lactone thereof.