The quantum yield of singlet oxygen (a1DELTA(g)), produced by energy transfer from the photosensitizer acridine, has been determined by two independent spectroscopic methods in solid polystyrene. Upon 355-nm pulsed-laser irradiation of acridine at 1.3 mJ/pulse, the O2(a1DELTA(g)) quantum yield in polystryene [phi(DELTA)PS(acridine) = 0.56 +/- 0.05] is slightly smaller than that in a liquid solvent analog such as benzene [phi(DELTA)benz(acridine) = 0.83 +/- 0.06]. These data indicate that the photosensitized yields of O2(a1DELTA(g)) in a solid polymer matrix can nevertheless be appreciable, which may be important for understanding the photodegradation of some polymeric materials or of dyes and other additives dissolved in those materials.