Abstract 2-Arylindenes (2),3-aryl-1,2-dihydronaphthalene(3),3-aryl-l,2-dihydro-4-methylnaphthalenes (4), 2-aryl-6-phenylindenes (5), 2-aryl-3-methyl-6-phenylindenes (6), 2-aryl-3,4-dihydro-6-phenylnaphthalene (7), 2-aryl-3,4-dihydro-lmethyl-6-phenylnaphthalenes(8), 2-aryl-3H-benz[e]indenes (9), 3-aryl-1,2-dihydrophenanthrenes (10), and 3-aryl-l,2-dihydro-4-methylphenanthren (11) in which the substituent aryl group was phenyl (a), l-naphthyl (b), 2-naphthyl (c), or 4-biphenylyl (d) were synthesized and tested as liquid scintillator solutes in toluene at various concentrations. Fluorescence spectra in toluene and ultraviolet absorption spectra were also determined. It was found that the compounds having the ethylenic double bond in a five-membered ring were better scintillators at low concentrations than those having the ethylenic double bond in a six-membered ring; however, at increased concentration the six-membered ring systems surpassed the fivemembered ring systems in scintillation efficiency. In addit...