We present optical and dielectric quantum-confinement effects for mesocrystals smaller than 30 Å of BaTiO3 and SrBi2Ta2O9 in uniform mesopores of the MCM-41 molecular sieve. For BaTiO3 mesocrystals we observed a blueshift in optical absorption edge from 3.02 to 3.35 eV, and a decrease in dielectric constant maximum temperature from 130 to 55 °C. For SrBi2Ta2O9 mesocrystals we also observed an increase in the optical absorption edge from 2.70 to 4.18 eV, and a decrease in the dielectric constant maximum temperature from 320 to 180 °C. The observed optical and dielectric quantum-confinement effects generally agree with the recognized consequence of reduced size. © 2000 American Institute of Physics.