The absorption and fluorescence properties of 29 specially selected methyl substituted benzene, naphthalene, anthracene, biphenyl, fluorene and p-oligophenylenes compounds are studied experimentally (at 293 K) and quantum-chemically. The fluorescence quantum yield, γ, and decay times, τf, for deaerated and non-deaerated solutions were measured. The oscillator strength, fe, natural lifetime, τf 0 and fluorescence and intersystem crossing rate constants, kf and kST, respectively, are calculated for each compound. The orbital nature of the lowest excited singlet state, S1, is determined. The investigation shows that the introduction of methyl groups onto aromatic compounds may produce different effects. For example, the symmetry and hence kST and kf may change. As a result, γ will also change. Steric hindrance, caused by the CH3 group(s) will decrease kf while increasing kST. In cases where the introduction of the methyl group leaves the symmetry unchanged, there is a slight increase in kST but a slight decrease in kf. This effect is cumulative (more CH3 groups lead to a greater decrease in γ) and is attributable to the torsional vibrations of the methyl groups. Cases where introduction of CH3 group(s) causes the inversion of 1La and Tβ levels are observed and explained. Such inversion is accompanied by the increase in kf and sometimes, by the significant decrease in kST, which leads to a dramatic increase in the fluorescence quantum yield. Such changes improve laser properties or can convert a non-lasing compound to a lasing one. It is predicted that the CH3 group(s) can cause the inversion of 1La and 1La or Sπ π* 1 and Tn π* 1 levels, which can change the nature of fluorescence or can change a nonfluorescent compound to a fluorescent one. The trends observed may be used to change fluorescence properties of an aromatic compound in the necessary direction without changing its π-system. © 2009 Elsevier B.V. All rights reserved.