In the time evolution of neutral meson systems, a perfect quantum coherence is usually assumed. The important quantities of the B(d) system, such as sin (2beta) and Delta m(d), are determined under this assumption. However, any real system interacts with its environment and this interaction can lead to decoherence. It is therefore desirable to re-examine the procedures of determination of sin(2beta) and Delta m(d) in meson systems with decoherence. We find that the present values of these two quantities are modulated by the decoherence parameter. Re-analysis of B(d) data from B-factories and LHCb can lead to a clean determination of decoherence parameter, sin(2beta) and Delta m(d).