The catalytic direct methanation of biogas to produce biomethane was conducted in a pilot plant with real biogas from a biogas plant in Zurich. Stable operation of the methanation system including a bubbling fluidised bed could be demonstrated for over 1100 h of regular operation with subsequent injection into the gas grid. An average methane yield of 96% was reached. During the long-duration experiment, the slow deactivation process was monitored and found to be only moderate. Organic sulphur compounds could be identified as the main source of deactivation. However, deactivation from coking could not be fully excluded. With appropriate measures in the gas cleaning unit, so that no sulphur compounds were measured subsequently (limit of detection: < 0.05 ppm of relevant components), the yield reduction went close to zero. Additionally, experimental results were compared to simulation results from a rate-based model presented elsewhere. Model predictions and experimental results are in accordance. The kinetics used in the rate-based model allowed a correct prediction of the optimum reaction temperature found by experimental results.