The BOLD signals related to brain activation is often nonlinear with change in TE. In contrast to extravascular component, the nonlinearity is attributable to intravascular component due to chemical exchange between plasma and deoxy-Hb. Recently, activity-evoked pH change on the brain has been demonstrated. Since the chemical exchange is often pH-dependent, the time for the chemical exchange would change. Thus, the two-compartment model that incorporates the change would be more accurate for estimation of parameters than the model with fixed exchange time. In this study, we measured the nonlinearity by multi-echo GRE-EPI and estimated parameters of the proposed model.