In this article a simple but effective method to calculate the quantum yield in heterogeneous photocatalysis is addressed. The amount of photons absorbed by coated photocatalyst was obtained with a portable radiometer, optical fiber (diameter 1 mm) and a purpose-built, fabricated accessory, as well as converted isopropyl alcohol, as the probe substance, with gas chromatography and a designed reactor system which determined a first order rate constant. Quantum yield was calculated under the assumptions that all the irradiated photons from one end of the fiber were either absorbed by the photocatalyst or transmitted to the other end and that the peak wavelength of the spectrum of the used UVBLB lamp represented the irradiated light. The resulting quantum yields with four different photocatalysts were in the range of 10-30% under specified reaction conditions of initial isopropyl alcohol concentration of 240 volume ppm, coated length of 10 cm, and initial transmitted light intensity of 20-25 μW/cm2.