Gas phase Beckmann rearrangement of cyclohexanone oxime is studied with B2O3/ZrO2 as the catalyst at 250-350 degrees C. It is shown that B2O3/ZrO2 is highly active and selective for the synthesis of E-caprolactam at 300-320 degrees C. Activity and selectivity of the B2O3/ZrO2 catalyst are compared with other boria catalysts supported on Al2O3, TiO2, SiO2, MgO, and HZSM-5 (Si/Al= 25) on an 10% B2O3 basis at 300 degrees C. At this boria load level, B2O3/ZrO2 produces the best catalysis for the lactam synthesis. The load level of boria affects the behavior of B2O3/ZrO2 catalyst: increasing the boria load (less than or equal to 20%) results in an increase of the lactam selectivity, but the lactam yield follows variation of the oxime conversion which shows a maximum at 10% boria load. Acidity and boria dispersion structure of the B2O3/ZrO2 catalysts are measured, respectively by NH3-TPD and IR spectroscopy. It is shown that the lactam yield is correlated with the number of intermediate strong acid sites that are responsible for ammonia desorption at 200-400 degrees C, The optimum boria load (10%) of the B2O3/ZrO2 catalyst corresponds to two overlayers of boria on the support. The first layer of boria interacts directly with the support surface with tetrahedral BO4 as the structure unit; the other layers of boria are constructed by trigonal BO3 units.