Abstract We report on persistent spectral hole-burning studies of thiazine and oxazine dyes encapsulated in an AlPO 4-5 aluminophosphate molecular sieve. A non-photochemical hole-burning mechanism is provided by small additional solvent molecules, e.g. glassy ethanol or amorphous water, in the pore system. Hole-burning efficiency and spontaneous hole-filling are in the typical range of glassy bulk systems. However, spectral diffusion is reduced drastically in the solvent-filled molecular sieve compared to the corresponding bulk glass. Based on previous work by the Fayer group [J. Chem. Phys. 92 (1990) 4125; Chem. Phys. Lett. 168 (1990) 371] this finding is discussed as an example of spatial restrictions to the solvation shell around the dye molecules.