  Magnetic excitations in an array of (VO)2P2O7 single crystals have been
measured using inelastic neutron scattering. Until now, (VO)2P2O7 has been
thought of as a two-leg antiferromagnetic Heisenberg spin ladder with chains
running in the a-direction. The present results show unequivocally that
(VO)2P2O7 is best described as an alternating spin-chain directed along the
crystallographic b-direction. In addition to the expected magnon with magnetic
zone-center energy gap DE = 3.1$ meV, a second excitation is observed at an
energy just below 2DE. The higher mode may be a triplet two-magnon bound state.
Numerical results in support of bound modes are presented.
