We have measured the magnetotransmission of thin film samples of YBa 2 Cu 3 O 7 in the normal state using circularly polarized light at frequencies from 15 to 180 cm 21 . The magneto-optical signal is consistent with the ac Hall effect of holes with a reduced scattering rate and an enhanced mass compared to the zero-field transport parameters. This result agrees with a simple high-frequency extension of models which have been developed to explain the anomalous dc Hall effect in the normal state of high-T c superconductors. PACS numbers: 78.20.Ls, 72.15.Gd, 74.72.Bk The nature of the ab-plane charge carrier dynamics in the normal state of YBa 2 Cu 3 O 7 is currently as intriguing a subject as the mechanism responsible for the superconductivity below T c . Hall measurements have revealed the charge carriers to be holes with a density related to the oxygen doping level, but there the resemblance to simple metals ends. A striking T 2 behavior of the inverse Hall angle cot u H s xx ͞s xy has been observed in many different types of samples [1][2][3][4]. In addition, the ac conductivity, derived from broadband optical reflectivity measurements [5,6], shows distinctly non-Drude-like behavior above 200 cm 21 . On the other hand, previous far-infrared (FIR) magnetotransmission measurements of YBa 2 Cu 3 O 7 in the superconducting state [7] have found a signal consistent with cyclotron resonance of free holes with a mass m c of ͑3.1 6 0.5͒m e .In this Letter we measure the FIR magneto-optical activity of YBa 2 Cu 3 O 7 thin films at 9 T and 95 K, where v c t ϳ 1͞40, so that we observe an overdamped cyclotron resonance response, or more appropriately the ac Hall effect. The signal is consistent with a holelike Hall effect, but fitting the data with a Drude model requires a smaller scattering rate and larger mass then the zero-field values. This result is understood by extending the ideas of Anderson [1,2] or Carrington et al. [3,4], used to explain the temperature dependence of the dc Hall effect, to high frequencies. In these theories, the Hall effect is dependent upon a scattering time and mass which are distinct from the ordinary transport quantities and are associated with spinons, or carriers near the corners of the 2D Fermi surface, respectively. Other phenomenological theories have also been proposed to explain the appearance of two scattering times in the charge dynamics of cuprate materials [8].Two types of thin film samples were used in these studies. High quality films of YBa 2 Cu 3 O 7 were produced by precursor deposition onto (100) LaAlO 3 substrates, followed by postannealing at 800 ± C in a wet oxygen atmosphere. Such films have proven to be of higher quality than typical laser-deposited samples, in terms of their degree of crystallinity and low defect density [9]. The superconducting transitions were characterized by dc resistivity and ac susceptibility measurements and found to be typically 92 6 0.3 K. Another class of samples were grown by pulsed laser deposition and in situ annealing on Si substrates ...