Abstract The far infrared transmission spectra of Ba 1− x K x BiO 3 ( x=0.4) thin films, grown by molecular beam epitaxy on MgO substrates, were measured in the frequency range of 15 cm −1 to 200 cm −1, and temperature range from 9K to 86K. In the normal state the transmission is independent of frequency. The transmission in the superconducting state approaches zero at zero frequency then rises with frequency to a peak whose position scales with the critical temperature and reduces as the temperature increases. The far infrared transmission curves are better described by strong coupling Eliashberg theory rather than weak coupling Mattis-Bardeen theory. From the Eliashberg calculation, which is based on the α 2 F( ω) data from tunneling measurements, the London penetration depth γ L (0) of a T c =18 K sample is found to be 5500±100 Å and the energy gap is 2 Δ(0)=4.0 kT c =50.1 cm −1 (6.2 meV). These results show that the strong coupling dirty limit gives distinctly different electrodynamics than the weak coupling dirty limit.