In symmetric multilevel inverters, there is a tradeoff between the output quality and the reliability and efficiency of the converter. New asymmetric and hybrid solutions, using different voltages and devices in various parts of the inverter, promise significant improvements for medium-voltage applications. This paper investigates such a hybrid asymmetric nine-level inverter. It consists of a three-phase three-level main inverter, with a two-leg two-level sub inverter in series with each phase (Fig. 1). To keep the power part simple and the efficiency high, the sub inverters have no feeding from the net and can only supply reactive power. But the nonsupplied intermediate-circuit capacitors form an unstable system. This paper proposes a control method to stabilize their voltages. Power balancing is guaranteed by varying the common-mode voltage, using an on-line nonlinear model-predictive controller. The controller predicts the system evolution as a function of the control inputs. A cost function of system and control quantities is iteratively minimized in real time, to find the optimal control to apply to the system. Simulations and measurements demonstrate stable behaviour in steady state and during transients. The originality of this paper is the application of nonlinear model-predictive control in power electronics.