Extensive ac-magnetic-permeability studies have been carried out on the irreversible behavior and flux dynamics in superconducting YBa2Cu307 crystals in external magnetic fields up to 7.5 T.From the analysis the irreversibility line B (T ) is found to scale as (1 -T /T, )", where n=l. 5 for fields higher than 1.5 T, and n=1 for lower fields. The exponents are sensitive to the choice of T,. The frequency dependence of the irreversibility temperature is carefully studied in the low-frequency region from 10 to 10 Hz and is found to be logarithmic in the whole frequency range. The small logarithmic slope increases with increasing external field. The analysis is done within a flux-creep picture using a single-relaxation-time form for the complex permeability, and