Magnetization measurements in La 0.225 Pr 0.40 Ca 0.375 MnO 3 allowed us to construct an H–T phase diagram of this prototype phase-separated manganite. Magnetic relaxation results clearly demonstrate the dynamic nature of the phase-separated state. Our results showed the existence of large relaxation effects, and the relative fraction of the coexisting phases rapidly changes as a function of time. Through a thermodynamic phenomenological model, it is possible to interpret the experimental results, both static and dynamic, of the phase-separated system. The phase diagram resulting from this model is presented, including the static and dynamic properties characteristics of the phase-separated state in manganites.