The dynamics of several prototypical molecular systems after excitation with femtosecond laser pulses at 155 nm has been studied in pump-probe experiments. The vacuum ultraviolet (VUV) pump pulses with a pulse width of 350-450 fs were generated by near-resonant four-wave difference frequency mixing in argon. The careful analysis of the time-dependent ion signals has allowed us to determine the lifetime of the excited molecular states down to about 30 fs. The extremely short lifetime of water molecules excited to the repulsive (A) over tilde state has been directly observed for the first time: tau(D)less than or equal to 20 fs. For molecular oxygen highly excited in the Schumann-Runge band, a decay time of 40 +/- 20 fs was obtained. The lifetimes of ethylene and chloroethylenes as well as of benzene and toluene reaching from 40 up to 180 fs are primarily caused by internal conversion. The decay times tau(D)=(1.9 +/-0.1) and tau(D)=(90 +/- 20) ps obtained for carbon disulfide and nitric oxide, respectively, are due to predissociation of the VUV excited states.