We prove an upper bound for the convergence rate of the homogenization limit e --> 0 for a linear transmission problem for a advection-diff??usion(-reaction) system posed in areas with low and high di??ffusivity, where e is a suitable scale parameter. On this way, we justify the formal homogenization asymptotics obtained by us earlier by proving an upper bound for the convergence rate (a corrector estimate). The main ingredients of the proof of the corrector estimate include integral estimates for rapidly oscillating functions with prescribed average, properties of the macroscopic reconstruction operators, energy bounds and extra two-scale regularity estimates. The whole procedure essentially relies on a good understanding of the analysis of the limit two-scale problem.