The surface structures of entrained air voids in hardened cement paste were examined by means of an environmental scanning electron microscope (ESEM). Three different pure anionic surfactants were used: sodium doclecyl sulfonate (SDS), sodium dodecyl-benzene sulfonate (SDBS), and sodium oleate (SO). It was observed that a distinct shell forms around most of the entrained air voids regardless of the surfactant used. It was also shown that some hydration products can grow into the air voids at very early ages and may fill considerable amount of the void volume. In addition to the microscopic evaluation, a modified foam index test and surface tension measurements were also conducted on the solutions with different mixtures. It was determined that since oleate molecules coming from SO were precipitated in the mix as calcium oleate, exiguous amount of air bubbles were stabilized in the paste; however, when SDS and SOBS were used, an acceptable amount of air bubbles were stabilized. (C) 2008 Elsevier Ltd. All rights reserved.