AbstractWe develop the technique of ionization parameter mapping (IPM) to probe the optical depth of Hii regions, applying our method to the Magellanic Clouds. Our results dramatically clarify the radiative transfer in these galaxies. Based on Sii, Oiii, and Hα imaging from the Magellanic Clouds Emission Line Survey, we find that the frequency of optically thin objects correlates strongly with Hα luminosity and correlates inversely with Hi column density. The aggregate escape fraction for the Lyman continuum is sufficient to ionize the diffuse, warm ionized medium, but the galactic escape fraction is dominated by the few largest Hii regions. The quantitative trends are similar in both the LMC and SMC in spite of their different star formation and Hi properties.