A new theory is proposed for quantum-mechanical multichannel scattering in a collinear three-body system. This simple problem is used to show that the principle of quantum determinism is generally violated and we are dealing with microirreversible quantum mechanics. Fundamental calculations of quantum (wave) chaos are presented for the first time for the elementary reaction Li+(FH)→(LiFH)*→LiF+H. ©1999 American Institute of Physics.