We have investigated the evolution of quantum Hall states in a GaAs-Al x Ga 1Ϫx As bilayer electron system by low-temperature magnetoresistivity measurements as the system was driven from a balanced to an offbalanced configuration. At low magnetic fields, odd integer filling factor quantum Hall states were observed on balance owing to the symmetric-antisymmetric tunneling gap. However, at high magnetic fields, in the regime of tunneling gap collapse, we observed anomalous quantum Hall states at vϭ2 off balance and vϭ3 on balance. At vϭ2, an energy gap was present all the way from the balanced configuration to far off balance, when only one quantum well was occupied. This is attributed to a transition from a spin-polarized state on balance to a spin-singlet state off balance, either by an abrupt exchange-driven phase transition or a continuous phase transition via a series of interlayer phase coherent states.

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