UV photodissociation of tungsten hexacarbonyl W(CO)6 has been studied in the molecular beam conditions using time-of-flight mass spectrometry and velocity map imaging. Irradiation of W(CO)6 by pulsed laser radiation at 266 nm results in the appearance of singly and doubly charged tungsten ions. The isotope composition of these ions deviates essentially from natural abundance with deviation being pulse energy-dependent. The velocity map images of the tungsten ions indicate proceeding of several, more than two, parallel channels (sequences of the one-photon processes) of photodissociation, giving rise to tungsten atoms. Isotope effect is assigned to appear in a one-photon bound-bound transition in W(CO) intermediate followed by its predissociation. In the model suggested, the final state of this transition is a vibronic state with excited vibrational mode of W-C stretching vibration. This vibrational excitation is responsible for isotopic shift in the location of the final state. The suggested model fits the observed isotopic composition quantitatively.