We report on GaAs/MCM-41 heterostructures synthesized by deposition of GaAs into the channels of MCM-41 using metalloorganic chemical vapor deposition. MCM-41 consists of an ordered array of silica tubules comprising pores with uniform and controllable diameter in the nanometer range. The GaAs/MCM-41 heterostructures show blue-shifted absorption and broad visible photoluminescence even at room temperature. The photoluminescence maximum depends on the MCM-41 pore diameter supporting formation of size-quantized semiconductor crystallites whose growth is restricted by the diameter of the pores of MCM-41. The shift in the photoluminescence spectra as a function of the excitation wavelength suggests a broad size distribution of the GaAs particles crystallized on the outside of MCM-41 and a relatively narrow size distribution of the GaAs particles inside the channels of MCM-41.