Magnetic and dielectric properties of perovskite manganites R0.5Sr0.5MnO3(R = Gd, Tb and Dy) have been investigated. DC and AC magnetic measurements showed short-range glassy magnetic ordering at T-g similar to 40 K. Such ordering was observed by neutron diffraction and is ascribable to the size mismatch of R3+ and Sr3+ settled randomly at the same crystallographic site. Dielectric constants for each material were similar to 1000-10,000 between similar to 50 and similar to 300 K and showed broad maximums above T-g. Dielectric dispersion showed poor coherency of the motion of polar regions, plausibly because of the size-mismatch effect; both the magnetic and dielectric properties of this system are governed by the randomness at the R/Sr site. The tan delta and EXAFS data suggest that the dielectric response is rooted in a transfer of the Mn-3d electrons. (C) 2010 Elsevier Ltd. All rights reserved.