Autothermal reforming of CH4 has been studied under both periodic and steady state conditions. The investigation was conducted over Co-NiO in a fluidised bed reactor at 873 K and 101.32 kPa. Cycle periods of 1-40 min were used whilst the cycle split, S-ox (with respect to the O-2-rich cycle) was varied from 0.1 to 0.9. Generally, CH4 oxidation stimulated CO formation, however, steam reforming yielded predominantly CO2 and H-2. Although O-2-rich cycling (S-ox greater than or equal to 0.5) was detrimental to H-2 formation, H2O-rich cycling resulted in a 15% improvement in steady state H-2 formation. Theoretical as well as experimental investigations pointed to a resonant frequency of about 6.7 mHz for CH4 oxidation to produce super steady state H-2 yields. By periodic operation, it is possible to tune H-2/CO ratios over the range 2.5-7 for the same feed composition. Interestingly, S-ox = 0.1 yielded the highest ratios, whereas the lowest ratios were attained at S-ox = 0.9. Periodic composition cycling introduces a more flexible approach to reactor operation - H-2/CO can be easily modulated by varying the cycle parameters - compared to steady state operation. (C) 2000 Elsevier Science B.V. All rights reserved.