  Motivated by the recent CPLEAR measurement on the time-reversal
non-invariance, we review the situation concerning the experimental
measurements of charge conjugation, parity violation and time reversibility, in
systems with non-Hermitean Hamiltonians. This includes in particular neutral
meson systems, like K0-barK0, D0-barD0 and B0-barB0. We discuss the formalism
that describes particle-antiparticle mixing and time evolution of states,
paying particular emphasis to the orthogonality conditions of incoming and
outgoing states. As a result, we confirm that the CPLEAR experiment makes a
direct measurement of violation of time-reversal without any assumption of
unitarity and CPT-violation. The asymmetry which signifies T-violation, is
found to be independent of time and decay processes.
