A detailed magnetostratigraphic investigation of the Agost section (Spain) containing the Cretaceous-Tertiary (K/T) boundary is reported. Thermal demagnetization--contrary to alternating field demagnetization--succeeds in revealing the polarity of the characteristic remanent magnetization, although an overlap in blocking temperature spectrum exists with a normal polarity, secondary magnetization component. The K/T boundary occurs at two-thirds from the base of a reversed polarity zone and by comparison with earlier results [1] this polarity zone is correlated to chron C29r.Linear regression of sedimentation rates in other sections with an established magnetostratigraphy and containing the K/T boundary yields an age of 66.45 Ma for this boundary. On the basis of this age and the magnetic reversal ages of the Berggren et al. [13]