Different strategies of inclusion were employed to obtain mesoporous channel-like silica MCM-41 with different content of terthiophene (3T), from almost isolated molecules to nearly filled nanochannels, and studied by optical measurements. In the first case, photoluminescence spectra of inclusion compounds were similar to dilute solution 3T emission, demonstrating the dispersive role of the silica matrix. In the second case, emission spectra resemble that of 3T crystals revealing the presence of an organic nanophase within the nanochannels. Simple estimations of the size of these nanoaggregates suggest that few tens of molecules are sufficient to give rise to bulk optical properties. (C) 2003 Elsevier B.V. All rights reserved.