The production of vitamin D3 is a pharmaceutically relevant process, producing high added‐value products. Precursors are extracts from vegetal origin but bearing mainly an E geometry in the 5,6 double bond. The synthesis of vitamin D3 (5‐E‐α‐calcidol) with the correct Z stereochemistry in the 5,6 double bond from the E isomer using anthracene and triethylamine (TEA) as the sensitizer system was studied from the kinetic and mechanistic point of view. The sensitized isomerization of E‐calcidol by irradiation of anthracene takes place only in deoxygenated solution and yields the Z isomer in ca 5% yield in the photostationary state. When TEA is added to the system, the E–Z reaction is not inhibited by oxygen any more, the quantum yield of photoisomerization to the Z isomer grows linearly with the concentration of E‐calcidol, while conversions higher than 95% to the Z isomer are reached in the photostationary state and E–Z quantum yields as high as 45 at [E‐calcidol] = 25 mm are reached. If TEA is replaced by 1,4‐diazabicyclo[2.2.2]octane, the reaction rate drops to one‐third at the same amine concentration. The observations can be explained by a quantum chain reaction mechanism. The high conversion achieved eliminates the need of isomer separation.