AbstractThe temperature dependence of the fluorescence quantum yield, the fluorescence decay time, and the triplet‐triplet absorption spectrum of 9‐methoxyanthracene have been measured in n‐heptane. The sum of the fluorescence quantum yield and the quantum yield for intersystem crossing of 9‐methoxyanthracene and two partially deuterated 9‐methoxyanthracenes at room temperature is only ∼0.60. While the fluorescence quantum yield in methyl methacrylate has nearly the same value as in n‐heptane, the fluorescence quantum yield is twice as high in the rigid matrix of PMMA. When passing from fluid solutions to the PMMA matrix, a dramatic increase in the fluorescence quantum yield can be demonstrated for the following meso‐substituted anthracenes: 9.‐tert.butyl‐, 9‐cyano, 10‐tert.butyl‐ and 9‐cyano,10‐methoxyanthracene. Anthracene, 9‐chloro‐ and 9‐phenylanthracene do not show neither this matrix effect nor an anomalous temperature dependence on the fluorescence quantum yield in solution. — The anomalous temperature dependence of the fluorescence quantum yield and fluorescence decay time and the increase in fluorescence quantum yield in the rigid PMMA matrix of 9‐methoxy‐and some meso‐substituted anthracenes indicate that at moderate temperatures a very fast S1→ S0internal conversion is opera