The influence of a magnetic field on the angular distribution of the atomic fragments resulting from photodissociation is studied both theoretically and experimentally. In the theoretical section we present a quantum mechanical description of photo-predissociation of diatomic molecules in a magnetic field, indicating the analogy with the classical Hanle effect observed in fluorescence, and showing how in a similar way lifetimes of predissociative states can be measured. The experimental section describes photodissociation experiments on the (4p)e1Pu(v = 0) Rydberg state of N2, where the dissociation process occurs in the presence of a magnetic field. We use the Hanle effect to determine the lifetimes of several rotational levels of this state. They range from 0.2 to 12 ns, in agreement with lower limits that were determined from high resolution spectroscopy. Lifetime information further elucidates the complex predissociation mechanism of the e 1Pu state.