The paper reports a comprehensive study of heat transfer, mass transfer, and entropy rate of humid air for a single solar still, concerning a typical summer day in a south east region of Tunisia. Transient humid air characteristics such as: partial pressures of components, temperature, humidity ratio, molecular diffusivity, and thermal diffusivity are determined and investigated. Entropy rate equation is derived and studied. Influences of humid air temperature and pressure on heat and mass transfer and on entropy rate are studied. It was found that the increase of water vapor partial pressure and humid air temperature are the best conditions for improving solar still productivity. Humid air can be regarded as a heat engine that generates kinetic energy by transporting energy from heated water surface to inner glass cover surface. Entropy rate of humid air largely depends on dry air entropy rate. The latter presents negative sign, meaning dissipation of convection heat transfer between evaporating and condensing surfaces during circulation. Inversely, entropy rate of water vapor has a positive sign, meaning that it receives energy during phase change by evaporation and transport by humid air from saline water surface towards the inner glass cover surface. (C) 2017 Elsevier B.V. All rights reserved.