AbstractFor a Lewis number one gas, the equations of energy and diffusion state that if local equilibrium exists in a fog region, fog may form in a continuous manner, but it may be vaporized only discontinuously at the boundary between fog and nonfog regions. The local rate of formation of fog is proportional to the square of the local gradient of temperature or concentration. A class of solutions to the transport equations has been obtained for problems in which no vaporization occurs at the fog boundary. Under these conditions, the temperature and concentration at the boundary of the fog region are both shown to be constant at values that depend only upon the initial and/or boundary values of temperature and concentration and upon the equilibrium curve.