We report for the first time an experimental application of the concept of a "black body" like reactor to measure quantum yields (Φ) of photochemical reactions in liquid - solid heterogeneous systems. A major advantage of this new method is its simplicity since the fractions of reflected and transmitted light are negligible due to reactor geometry and high optical density of the heterogeneous systems. The average quantum yield of a test reaction (phenol photodegradation) over TiO₂ (Degussa P25) as determined by this method was 0.14, identical to the quantum yield measured earlier for this same reaction under similar conditions by Salinaro and Serpone [Pure Appl. Chem. 1999, 71, 303]. We also report the quantum yield of phenol photodegradation over N-doped TiO₂ during photoexcitation at the fundamental absorption band (λ = 365 nm; Φ = 0.12) and at the N-doping induced extrinsic absorption band (λ = 436 nm; Φ = 0.08) of the photocatalyst.