The feasibility of using laser photodetachment as a means for isobar suppression in accelerator mass spectrometry has been investigated for the special case of HfF5-/WF5-. A method for absolute photodetachment cross section measurements was applied and the cross sections of tungsten pentafluoride and hafnium pentafluoride negative ions were measured. The measurements indicate that the photodetachment cross section for WF5- is at least 100 times larger than for HfF5- at the wavelength of the fourth harmonic of the Nd:YAG laser at 266 nm. The absolute cross section for WF5- at this photon energy was found to be (2.8 ± 0.3) × 10-18 cm2, while an upper limit of 2 × 10-20 cm2 was obtained for the HfF5- cross section. The measured cross sections indicate that an optical filtering scheme for isobar suppression in accelerator mass spectrometry for the case of Hf182 should be feasible.