The main impurities produced in the Beckmann rearrangement of cyclohexanone oxime (CHO) to yield e-caprolactam (CPL) in oleum were identified as cyclohexanone, 2-cyclohexen-1-one, 2-hydroxycyclohexan-l-one, 1,2-cyclohexanedione, and 1,2,3,4,6,7,8,9-octahydrophenazine. The effects of several operating parameters on impurity formation were also investigated, including stirrer speed, acid/oxime ratio, SO3 concentration, temperature, and CHO concentration. Although impurity formation is unavoidable in this process, the results indicated that impurity formation was generally reduced with an enhanced mixing performance, a higher acid/oxime ratio, and a higher SO3 concentration, thereby indicating that these three factors play a key role in determining the impurities resulting from this transformation. Potential formation pathways for the impurities were also proposed and discussed based on the experimental results. These results are therefore expected to contribute to the development of guidelines for process design and reaction-condition optimization for the industrial production of CPL.