  We study the time evolution of the entangled kaon system by considering the
Liouville - von Neumann equation with an additional term which allows for
decoherence. We choose as generators of decoherence the projectors to the
2-particle eigenstates of the Hamiltonian. Then we compare this model with the
data of the CPLEAR experiment and find in this way an upper bound on the
strength $\lambda$ of the decoherence. We also relate $\lambda$ to an effective
decoherence parameter $\zeta$ considered previously in literature. Finally we
discuss our model in the light of different measures of entanglement, i.e. the
von Neumann entropy $S$, the entanglement of formation $E$ and the concurrence
$C$, and we relate the decoherence parameter $\zeta$ to the loss of
entanglement: $1 - E$.
