The trans{cis photoisomerization of azobenzene{containing ma- terials is key to a number of photomechanical applications, but the actual conversion mechanism in condensed phases is still largely un- known. Here we have studied the (n ->pi*) isomerization in vacuum and in various solvents via a modied molecular dynamics simulation adopting an ab initio torsion{inversion force eld in the ground and excited states while allowing for electronic transitions and for a stochastic decay to the fundamental state. We have determined the trans{cis photoisomerization quantum yield and decay times in various solvents (n{hexane, anisole, toluene, ethanol and ethylene glycol), obtaining results comparable with the experimental ones where available. We nd a profound dierence between the isomerization mechanism in vacuum and in solution, with the often neglected mixed torsional{inversion pathway being the most important in solvents