The free radical terpolymerization of pentamethylcyclopentaarsine (cyclo-(MA)(5)), phenylacetylene (PA), and various vinyl monomers in toluene is described. The terpolymerization of cyclo-(MA)5, PA, and styrene (St) in the molar ratio 1:5:5 with a catalytic amount of AIBN at 60 degreesC for 8 h provided a polymer in 38% yield after reprecipitation. The terpolymer was soluble in common organic solvents such as tetrahydrofuran, chloroform, benzene, and toluene. In the GPC analysis (chloroform, PSt standards), the terpolymer showed a single peak, and the number-average molecular weight was estimated to be 10 200. The structure of the terpolymer obtained was compared with those of poly(vinylene-arsine) and polystyrene by use of H-1 NMR and C-13 NMR and found to have two block units, [poly(MA-alt-PA)poly(St)]. The integral ratio of the peak area in the H-1 NMR spectrum revealed the terpolymer composition, MA:PA:St = 1:0.99:0.56. The terpolymer showed fluorescence properties attributable to the n-pi* transition in the poly(MA-alt-PA) block. The terpolymerization of cyclo-(MA)(5), PA, and St in various feed ratios was carried out in the presence of AIBN to yield the terpolymers which possessed poly(MA-alt-PA) and poly(St) blocks with various compositions. The terpolymer with the higher content of St unit exhibited a more blue-shifted emission peak. The radical terpolymerization of methyl methacrylate (MMA) with cyclo-(MA)(5) and PA also yielded the corresponding terpolymer with a poly(MA-alt-PA) unit and a poly(MMA) unit, while the terpolymerization employing vinyl acetate (VA) as a vinyl monomer produced poly(vinylene-arsine) with no VA segment. The radical reaction of 2,3-dimethyl-1,3-butadiene (DB) with cyclo-(AsMe)(5) and PA formed a different type of terpolymer which had alternating sequences of AsMe and PA or DB, e.g., [(MA-PA)-(MA-DB)](n).