Nicotinium dichromate (NDC) oxidation of mandelic acid and P-chloromandelic acid in aqueous acetic acid medium in presence of sulphuric acid lead to the formation of corresponding benzaldehyde and P-chlorobenzaldehyde as the main products. The reaction follows fractional order with respect to substrate and first-order with respect to the oxidant. Increase in percentage of acetic acid inhibited the rate of reaction. The reaction is catalysed by acid. The proposed intermediate involves a complex mechanism. The derived rate law was verified graphically. The thermodynamic and activation parameters for the oxidation have been determined and discussed.