We demonstrate here that new phenomena can occur in mesoscopic commensurate
systems of strongly correlated electrons: tunneling-driven quantum phase t
ransitions. To exemplify, we present CDW-to-CDW (SDW-to-SDW) transitions be
tween charge (spin) density wave states of different symmetries in quantum
rings described by an extended Hubbard model. Thesde transitions can be stu
died experimentally e.g. by optical absorption.